ADU Glossary

A secondary housing unit on a single-family or multifamily residential lot — also called a granny flat, in-law unit, or backyard cottage. Must have independent living facilities: kitchen, bathroom, and sleeping area.

An ADU sharing at least one wall with the primary dwelling. Often created through additions or by converting an attached garage or bonus room.

A 2019 California law streamlining ADU permitting, limiting local review to 60 days and eliminating several barriers that previously delayed ADU approvals.

A 2022 California law prohibiting local agencies from requiring applicants to correct existing code violations unrelated to the ADU as a condition of permit approval.

Design features accommodating people with disabilities, including wider doorways (32–36"), grab bars, zero-step entries, and lever-style hardware. Expands rental market and future-proofs the unit.

The scheduled repayment of a loan through regular principal and interest payments over the loan term. Used when financing ADU construction through a home equity loan or mortgage.

A bolt embedded in concrete foundations to secure the sill plate and connect wood framing to the foundation. Required by code at specific intervals (typically every 6 feet).

A licensed professional's assessment of property value. ADUs typically increase property value, but the appraisal method (cost vs. income vs. sales comparison) affects the final number significantly.

Drawings documenting the actual constructed conditions of a building, reflecting any changes made during construction from the original approved plans.

A government official who determines property values for tax purposes. Completing an ADU typically triggers reassessment of the improvement value only — not the entire property under Prop 13.

A plumbing device preventing contaminated water from flowing backward into the clean water supply. Required at irrigation connections and per code when adding new water connections for an ADU.

A wall that supports structural loads from above, including floors, roofs, or other walls. Cannot be removed without providing alternative structural support designed by a structural engineer.

Regulations governing the design, construction, and occupancy of buildings to ensure safety. California uses the California Building Standards Code (Title 24), updated every three years.

Official approval from the local building department authorizing construction work. California requires ADU permit decisions within 60 days of a complete application submittal.

The physical barrier between the conditioned interior and unconditioned exterior — walls, roof, windows, doors, and foundation. Tighter envelopes reduce energy bills and improve comfort.

Development permitted under zoning without discretionary approval (no hearings, design review panels, or neighbor votes). California ADU law requires by-right ministerial approval for qualifying ADUs.

Short-term financing bridging the gap between ADU construction costs and permanent funding, typically at higher interest rates. Used when permanent financing isn't available until the ADU is complete.

A diagnostic test using a calibrated fan to measure a building's air-tightness. Often required by California's Title 24 energy code to verify that new ADUs meet airtightness standards.

California Green Building Standards Code (Part 11 of Title 24) — the first state-mandated green building code in the U.S. Sets requirements for water efficiency, materials, and indoor air quality.

A document from the building department confirming the ADU complies with all codes and is safe for occupancy. Required before tenants can legally move in.

A written amendment to the construction contract documenting changes to scope, schedule, or cost. Should always be signed by both owner and contractor before work proceeds.

Replacing an existing mortgage with a new, larger loan and receiving the difference in cash. One of the most common ways homeowners fund ADU construction, especially with high existing equity.

An ADU created by converting existing space — a garage, basement, attic, or portion of the primary dwelling. Often the most cost-effective ADU type because the structure already exists.

A shallow unfinished space between the first floor and ground, providing utility access. Must be properly ventilated and covered with a vapor barrier to prevent moisture damage.

The California agency licensing and regulating the construction industry. Always verify your ADU contractor holds a current CSLB license at cslb.ca.gov before signing any contract.

A reserve — typically 10–20% of total project cost — set aside for unexpected expenses during ADU construction. Skipping contingency is one of the most common homeowner mistakes.

A real estate metric dividing net operating income by property value, used to evaluate ADU rental return. A $30k/year ADU on a $300k investment yields a 10% cap rate.

A row of windows set high in a wall above eye level, bringing natural light into the interior without sacrificing wall space or privacy. Popular in modern ADU designs.

A limitation on property use recorded in the deed. Some California ADU laws override private CC&R deed restrictions that would prohibit ADU construction.

A stand-alone ADU with no shared walls with the primary dwelling. California allows detached ADUs up to 1,200 sq ft (or the size of the primary dwelling, whichever is less).

Charges imposed by local governments to fund infrastructure. California prohibits impact fees on ADUs under 750 sq ft and limits fees proportionally for larger units.

A project delivery method where a single firm handles both design and construction. Often faster and better coordinated for ADU projects — but compare bids, as pricing varies widely.

A payment plan tied to construction milestones releasing funds in stages as ADU work is completed. Protects homeowners from paying for work not yet performed.

The standard interior wall material made of gypsum pressed between paper sheets. 1/2" for walls; 5/8" Type X required for fire-rated assemblies (e.g., near property lines).

A building containing two separate dwelling units. SB 9 (2022) allows lot splits and duplexes on qualifying single-family lots, potentially enabling up to four units per parcel.

A building or portion designed for residential occupancy with independent living, cooking, and sanitation. Each ADU is a legally separate dwelling unit from the primary residence.

A right to use another's property for a specific purpose — utilities, drainage, access. ADUs typically cannot be built within recorded easements; check your title report before planning.

A window sized for emergency escape, required in every ADU bedroom. Minimum 5.7 sq ft of net clear opening, max 44" sill height, min 20" wide and 24" tall.

California's building energy efficiency standards, among the most stringent in the nation. Governs insulation, windows, HVAC, water heating, lighting, and solar requirements for new ADUs.

The difference between a property's market value and outstanding mortgage balance. Most homeowners tap equity through a HELOC or cash-out refinance to fund ADU construction.

Manufactured wood products — LVL, I-joists, glulam — engineered to be stronger and more dimensionally stable than solid lumber. Common in ADU floor and roof systems.

A ventilation device that exchanges stale indoor air with fresh outdoor air while recovering both heat and moisture. Ideal for tight, well-insulated ADUs to maintain air quality efficiently.

A gap between building elements allowing thermal expansion, settling, or seismic movement without damage. Required where new ADU construction meets existing structures.

The visible outer surface — siding, stucco, paint, trim. In fire hazard severity zones, certain non-combustible or ignition-resistant finishes are required by California building code.

A preliminary analysis determining whether an ADU is buildable on a specific property, considering zoning, setbacks, lot coverage, slope, utility capacity, and estimated costs.

The duration a wall, floor, or roof assembly can resist fire, expressed in hours. One-hour rated construction is often required for ADU walls within 3–5 feet of a property line.

Automatic fire suppression systems. California requires sprinklers in new ADUs only when the primary dwelling already has them — check local ordinances, as some cities are stricter.

The ratio of total building floor area to lot area. Many California cities exempt ADUs from FAR calculations per state law, making projects feasible on smaller lots.

The structural system transferring building loads to the earth. Common ADU foundations: slab-on-grade (most cost-effective), raised floor on piers, and continuous perimeter footing.

The structural skeleton of a building using wood or steel studs, joists, and rafters. Platform framing — where each floor acts as its own platform — is the standard for residential ADUs.

A gravel-filled trench with a perforated pipe redirecting groundwater away from the ADU foundation, preventing water intrusion and structural damage over time.

The last building department inspection verifying all work is complete and code-compliant. The certificate of occupancy is issued after a passing final inspection.

Converting an existing attached or detached garage into an ADU. California law prohibits requiring replacement parking when a garage is converted. Often the lowest-cost ADU type.

A licensed professional managing the entire ADU project — hiring subs, scheduling inspections, and ensuring work meets plans. Must hold a current CSLB license in California.

A soils engineering study analyzing ground conditions to guide foundation design. Typically required on hillside lots, expansive clay soils, or fill areas before ADU construction.

An electrical safety device shutting off power when it detects current leakage to ground. Required in ADU kitchens, bathrooms, garages, outdoor areas, and within 6 ft of any sink.

A colloquial term for an ADU, originally referring to small homes for aging parents. Now used interchangeably with ADU, in-law unit, accessory apartment, and backyard cottage.

A system recycling wastewater from showers, sinks, and laundry (excluding toilets) for landscape irrigation. California allows permitted greywater systems, which can reduce water bills.

An unpermitted dwelling unit that exists without official approval. California's ADU amnesty and permit correction programs offer a path to legalization for many gray market units.

Space used for living, sleeping, eating, or cooking. Must meet minimum 7-foot ceiling height, 8% glazing-to-floor-area for natural light, and natural or mechanical ventilation requirements.

An HVAC system moving heat between indoor and outdoor environments for both heating and cooling. Highly efficient and increasingly mandated in California new construction for ADUs.

A revolving credit line secured by home equity, commonly used for ADU construction. Draw funds as needed, pay interest only on what you use — but rates are variable.

A private governing organization for a residential community. California law limits HOA ability to prohibit ADUs, but HOAs may still require reasonable architectural standards and design review.

A synthetic weather-resistant barrier applied to exterior walls behind siding, providing air/water resistance while allowing moisture vapor to escape. Examples: Tyvek, Typar, Zip System.

Heating, Ventilation, and Air Conditioning. ADU options include ductless mini-splits (most common), through-wall units, and central HVAC shared with or separate from the primary dwelling.

A required component of California city/county general plans addressing housing needs and RHNA allocations. ADU-friendly policies are an essential tool for cities to meet state housing goals.

Charges imposed on new development to fund public infrastructure. California prohibits impact fees for ADUs under 750 sq ft and requires proportional fees for larger ADUs.

A common term for an ADU, traditionally used to describe secondary housing for extended family. Now synonymous with ADU, granny flat, and accessory apartment.

Official examination of construction work by the building department. Required milestones include foundation, rough framing, rough MEP (mechanical/electrical/plumbing), insulation, and final.

An ADU created within the existing footprint of a primary dwelling — converting a basement, attic, or portion of the house. Requires separate entrance per California ADU law.

Hollow foam blocks stacked into walls and filled with concrete, creating a well-insulated, high-mass wall system. Offers excellent energy performance and disaster resistance for ADUs.

Any surface preventing water infiltration — rooftops, concrete, asphalt. Adding impervious area for an ADU may trigger stormwater management requirements in many jurisdictions.

A small ADU of 500 sq ft or less created within the walls of an existing single-family home. Must have a separate entrance and efficiency kitchen; the bathroom may be shared with the primary unit.

A horizontal structural member supporting floors or ceilings, spanning between beams or bearing walls. Typically spaced 12", 16", or 24" on center depending on span and load.

An electrical enclosure where wires are connected. Must remain accessible and properly covered — never bury a junction box inside a finished wall or ceiling.

Material used to fill seams between drywall panels and cover fastener holes. Applied in multiple thin coats and sanded smooth before painting.

A full ADU requires a complete kitchen with sink, cooking appliance, and refrigerator. JADUs may use an efficiency kitchen — a sink and cooking surface without a full-size range.

A short wall (typically under 4 feet) where a sloped roof meets the floor in an attic conversion ADU. Creates usable space under low ceilings and often conceals knee wall storage.

A unit of electrical power equal to 1,000 watts. Solar panel systems are rated in kW; electrical panel capacity is often expressed in kW. ADUs commonly add 1–2 kW of solar.

Horizontal forces on a structure — primarily from wind and earthquakes. Resisted by shear walls, diagonal bracing, or moment frames. Critical for ADUs in California's seismic zones.

A legal claim against property as security for payment. Subcontractors and suppliers can file mechanics' liens if unpaid. Always collect signed lien waivers before final payment.

The percentage of a lot covered by buildings and impermeable surfaces. Local zoning sets maximums; California law often exempts or provides allowances for ADUs in lot coverage calculations.

The legal boundary of a property parcel. Setbacks are measured from lot lines. A licensed surveyor can mark exact lot lines if there's uncertainty about where your property ends.

Window glass with a microscopically thin metallic coating reflecting radiant heat. Required by California's Title 24 energy code for most ADU window applications to reduce cooling loads.

Engineered lumber made from bonded wood veneers, used for headers, beams, and rim boards. Stronger and more dimensionally stable than solid sawn lumber of the same size.

A document from a contractor or supplier relinquishing the right to file a mechanics' lien. Collect conditional waivers upon each progress payment and unconditional waivers upon final payment.

A ductless HVAC system with an outdoor compressor and one or more indoor air handlers. The most popular ADU HVAC choice due to efficiency, compact size, and installation flexibility.

Permit approval based on objective standards without discretionary judgment. California mandates ministerial ADU approval within 60 days for applications meeting all objective requirements.

Building method where sections are manufactured off-site and assembled on the property. Must meet the same building codes as site-built ADUs. Typically 20–30% faster than stick-built.

Material preventing water vapor transmission, installed in crawl spaces, under slabs, and in wall assemblies. Critical for preventing mold, wood rot, and structural damage in ADUs.

The three building systems providing thermal comfort, power, and water. MEP costs typically represent 25–35% of ADU construction budgets and require separate permits in most jurisdictions.

ADUs permitted on multifamily properties under California law. Allows conversion of non-livable space (storage, laundry rooms) plus new detached ADUs, with specific limits per building.

The U.S. standard for safe electrical installation, adopted (with amendments) by most jurisdictions. California uses the NEC as the basis for its own electrical code.

An ADU that produces as much energy as it consumes annually — typically achieved through solar panels, battery storage, superior insulation, and high-efficiency systems.

A billing arrangement crediting solar energy system owners for electricity they send to the grid. Applies to ADUs with photovoltaic panels. California NEM 3.0 changed credit rates in 2023.

A structure that was legal when built but no longer complies with current zoning. California ADU law often allows nonconforming structures to be converted to ADUs without full compliance.

Building codes require glazing area (windows/skylights) equal to at least 8% of the floor area of each habitable room for natural light. Mechanical lighting doesn't substitute.

Measurement from the center of one framing member to the center of the next. Standard stud spacing is 16" OC; 24" OC is used in advanced framing to increase insulation space.

A layout with minimal walls between living areas. Popular in ADUs to maximize perceived space — but requires careful structural planning since some walls may be load-bearing.

A property owner acting as their own general contractor. Requires signing a state disclosure acknowledging liability. May affect insurance coverage and financing — consult your lender first.

A prior rule requiring homeowners to live on the property when renting an ADU. California law abolished owner-occupancy requirements for ADUs statewide through 2025.

A building permit approved during a single visit, available in some jurisdictions for simple ADU projects using pre-approved plan sets. Dramatically reduces wait times.

A legally defined piece of real property. ADUs must be located on the same parcel as the primary dwelling unless the parcel is subdivided under SB 9.

California prohibits cities from requiring ADU parking in many scenarios: within half a mile of transit, in historic districts, within one block of a car-share, or when converting existing structures.

Pre-designed ADU plans vetted to meet common code requirements, available through many California cities. Reduce design costs and accelerate permitting — check if your city offers them.

Review of construction documents by the building department for code compliance before permit issuance. California limits ADU plan check time to 60 days from complete application submittal.

The main residential structure on a property to which the ADU is accessory. California law requires a primary dwelling to exist or be proposed concurrently with the ADU application.

ADU components manufactured off-site and assembled on location — including modular, panelized, and container-based systems. Growing market segment offering faster timelines and predictable pricing.

Flexible cross-linked polyethylene tubing for water supply lines. Popular in ADU plumbing for ease of installation, freeze resistance, and corrosion resistance. Approved in all California jurisdictions.

A detailed list of minor defects, incomplete items, and touch-ups that must be corrected by the contractor before final payment. Walk through with your contractor systematically before releasing the final check.

A lender's conditional commitment to finance up to a specified amount based on financial documentation review. Obtaining pre-approval before ADU design saves time and sets a realistic budget.

Systematic processes ensuring construction meets plans and specifications. Includes milestone inspections, material testing, and photo documentation throughout the ADU build process.

The process of measuring and listing all materials needed from construction drawings. Used for accurate cost estimating and material ordering — a key step before finalizing ADU bids.

A measure of thermal resistance in insulation. Higher R-values = better performance. California's Title 24 specifies minimum R-values by climate zone (e.g., R-38 attic in Zone 6).

The required distance between a structure and the rear property line. California ADU law limits rear setbacks to a maximum of 4 feet for detached ADUs meeting height requirements.

Steel bars embedded in concrete to increase tensile strength. Diameter, spacing, and placement are designed by a structural engineer based on loads, soil, and seismic requirements.

Revenue from renting an ADU, which can offset construction costs and mortgage payments. Lenders may count up to 75% of projected ADU rental income to help qualify for financing.

A structure holding back soil at different elevations. Walls over 4 feet typically require a building permit and structural engineering. Critical for hillside ADU projects.

Financial gain relative to investment cost. ADUs in California typically see strong ROI through rental income (often $1,500–$3,000+/month) and property value increases of $100k–$300k+.

Installation of mechanical, electrical, and plumbing systems within walls and floors before covering with finish materials. Must pass inspection before walls are closed up.

California law (2022) allowing lot splits and duplexes on qualifying single-family lots in urban areas. Combined with ADU/JADU rights, can enable up to four units per parcel.

2019 California law that eliminated owner-occupancy requirements, reduced impact fees for small ADUs, and further streamlined the ADU permitting process statewide.

A 2022 California law increasing allowable ADU heights, expanding ADU rights on multifamily properties, and further limiting local governments' ability to restrict ADU construction.

A 2024 California law that further streamlined ADU regulations, including allowing detached ADUs to be sold separately from the primary dwelling as condominiums under specific conditions.

A detailed description of all work, materials, and outcomes under a construction contract. The more specific the scope, the less room for disputes and surprise cost increases.

An on-site sewage treatment system using a tank and leach field. ADUs on septic properties typically require a system capacity evaluation and may need upgrades or expansion.

The required distance between a structure and property lines. California caps ADU setbacks at 4 feet from side and rear property lines for compliant projects, overriding stricter local rules.

A wall designed to resist lateral forces from wind and earthquakes through rigid sheathing and specific fastening patterns. Locations and nailing schedules must be engineered and inspected.

Renting an ADU for less than 30 days, typically through platforms like Airbnb or VRBO. Many California cities restrict or prohibit STRs — always verify local ordinances before planning STR income.

A drawing showing property boundaries, existing/proposed structures, setbacks, utilities, and drainage. Required for all ADU permit applications. Often prepared by a designer or civil engineer.

A concrete foundation poured directly on prepared ground, serving as both foundation and floor. The most cost-effective ADU foundation type for level sites in mild California climates.

A rating measuring how well a wall or floor assembly blocks airborne sound. STC 50+ walls provide good privacy between an ADU and the primary dwelling. Consider upgrading beyond code minimums.

A licensed professional who designs structural systems ensuring they safely resist loads. Required for most California ADU projects to stamp foundation, framing, and shear wall designs.

A secondary electrical panel fed from the main panel, distributing power to ADU circuits. Commonly 100–200 amps for a new ADU. Location should minimize feeder wire length.

A professional measurement and mapping of property boundaries and features by a licensed land surveyor. Recommended before ADU design to confirm exact setback distances from lot lines.

An on-demand water heating system heating water only when needed. Popular for ADUs for its compact size and energy efficiency — requires adequate gas supply or dedicated 240V circuit.

California Building Standards Code. Part 6 covers energy efficiency; Part 11 is CALGreen. New ADUs must comply with the version adopted when the permit application is submitted.

The shape and elevation of land. Steep slopes add cost to ADU foundations (deeper piers, retaining walls) and may trigger additional grading permits and geotechnical requirements.

A prefabricated structural frame for roofs or floors composed of triangulated members. Faster and often cheaper to install than stick-built framing, but limits future attic conversion.

A construction approach where the contractor delivers the ADU fully complete and ready for occupancy — including fixtures, appliances, and landscaping. Higher upfront cost but lowest homeowner effort.

A construction agreement where the homeowner pays actual labor hours and material costs plus a contractor markup. Flexible but carries budget uncertainty — request a "not-to-exceed" cap.

Insurance protecting property owners and lenders against claims arising from defects in property title. Recommended when purchasing a lot or property for ADU development.

A measure of heat flow through building materials — the inverse of R-value. Lower U-factors mean better window insulation. California's Title 24 requires U-factors of 0.30 or lower for most ADU windows.

Work done without required permits. Creates liability, complications at property sale, and potential insurance issues. California ADU amnesty programs offer paths to legalization.

A subdivision process created by SB 9 allowing qualifying single-family parcels in urban areas to divide into two lots. Each lot may host a primary dwelling plus an ADU and JADU.

The point where ADU systems connect to public utilities. California prohibits requiring separate utility connections for ADUs — they may share meters with the primary dwelling.

Design principles creating spaces usable by people of all ages and abilities. Wider doors (36"), lever hardware, curbless showers, and accessible bathrooms expand ADU marketability significantly.

Material resisting moisture vapor diffusion through building assemblies. In California's mild climate, typically placed under slabs and in crawl spaces rather than within wall assemblies.

An exception to zoning regulations granted when strict compliance creates undue hardship. California's ADU laws reduce variance needs by setting statewide minimum standards that override stricter local rules.

Exchange of indoor and outdoor air to maintain air quality. ADU habitable rooms need natural ventilation (4% openable area) or mechanical ventilation meeting ASHRAE 62.2 standards.

Chemicals released as gases from paints, adhesives, and flooring. CALGreen limits VOC content in products used in new construction. Choose low-VOC products for a healthier ADU interior.

Analyzing a design to reduce costs while maintaining quality and function. Common ADU value engineering: simplifying roof lines, standardizing room dimensions, and choosing modular cabinet sizes.

A device measuring water consumption. California prohibits requiring separate water meters for ADUs, allowing shared service with the primary dwelling to avoid costly new connection fees.

Treatments and materials preventing water penetration into building assemblies. Critical for ADU foundations, below-grade walls, shower assemblies, and flat roof membranes.

A metal flashing at the bottom of stucco walls allowing trapped moisture to drain while providing a clean termination above grade. Required by California code on all stucco-clad ADUs.

Areas where development borders wildland vegetation. ADUs in WUI zones must use ignition-resistant exterior materials — Class A roofing, ember-resistant vents, and non-combustible eave assemblies.

A kitchen design concept: the combined distance between refrigerator, sink, and range should total 13–26 feet. Compact ADU kitchens often achieve excellent work triangles in smaller footprints.

A thorough inspection of the completed ADU by the homeowner and contractor to identify defects and incomplete items. Document everything in writing as your punchlist before final payment.

Diagonal cross-bracing providing lateral stability to structural frames. Common under raised floor ADUs and in steel-frame construction. Visually apparent in exposed structural designs.

Landscaping designed to minimize water use through drought-tolerant plants and efficient drip irrigation. Encouraged or required around ADUs in many California jurisdictions as part of grading plans.

In zoning terms, the open space required between a building and property lines. Front, side, and rear yards are defined by setback rules. California ADU law caps yard requirements for ADUs.

The stress level at which a structural material permanently deforms. Engineers design below the yield point with safety factors to ensure ADU structural members remain elastic under design loads.

Construction that extends to the property line with no side setback. Typically requires a one-hour fire-rated wall with no openings. May be allowed for specific ADU configurations under local codes.

Local regulations dividing land into districts with specific permitted uses and development standards. California ADU laws override local zoning restrictions that would otherwise prohibit or excessively limit ADUs.

Verification that a proposed ADU project complies with zoning regulations, typically the first step before a building permit application is reviewed.

A section of a building with independent temperature control. ADUs typically constitute a single HVAC zone with their own thermostat and equipment, independent from the primary dwelling.

An official map showing designated land use zones within a jurisdiction. Use your city's zoning map to confirm your property's zone and which ADU types and sizes are allowed by right.