What is an ADU?
An accessory dwelling unit or ADU is a home that is built on a parcel with an existing single family home. These are often converted from garages or other existing accessory spaces and are limited to smaller footprints, fewer bedrooms, and can also be built as separate apartments inside existing homes. They can be used to provide rental income, as guest spaces, family homes for elderly parents, or as bonus and office spaces. Junior ADUs or JADUs differ from typical ADUs in that they must exist within the existing home or an attached accessory structure such as a garage. They may share systems with the primary dwelling including bathrooms and can use efficiency kitchens. An ADU can be detached or converted from existing space and a JADU must be converted from existing space.
Number of ADUs allowed
Generally, state law allows for one ADU and one JADU on almost all single family residential lots. Single-family homeowners in California are eligible to add one ADU as well one JADU on their property if they meet the minimum space requirements for setbacks and lot coverage, as well as other regulations. JADU’s can only be built on owner-occupied properties. Owner occupancy requirements are waived for ADUs until 2025 in most cases, but still apply to JADUs.
Multifamily units are eligible for at least one ADU converted from non-living space, and up to 25% of the total number of multifamily units currently on the lot. They also are eligible for at least one and up to two detached units, granting that they meet height and setback requirements.
Basic ADU Regulations
Building an ADU, from ADU design to construction
Design
ADUs, like all residential construction, must meet state building code requirements, and some municipalities have additional design standards that limit height and square footage. Almost all ADUs are required to match the design but not the materials of the primary home. For example, a Spanish Style home built with adobe and clay roofing tiles may be able to precast concrete walls and use non-clay (most often metal or vinyl tiles) so long as the style matches. This varies by municipality and depends on the reason for requiring clay tiles, which are often used to prevent fires from spreading via ash fall. Similarly the windows of the primary home may be the original windows and single pane, this would not be required in your new ADU. The interior of the ADU doesn't need to match the style of the primary home. While standard interior layouts and appliances can save a great deal of money down the road, homeowners are able to design ADU interiors in whatever style that suits them. Additional regional building standards for fire, flooding, or earthquake hazards may also apply.
Flood zones: If you live near a creek, stream, or river (or in a coastal flood zone near a bay, estuary or coastline) you will want to check FEMA flood requirements which require elevation of habitable spaces above a base flood elevation determined by the 100 year flood events for the waterway you live near. In most cases, if you live in the floodplain your ADU will require at a minimum an 18 inch stem-wall with flood vents. Coastal ADU projects in coastal flood zones may require significant elevation as well as breakaway walls if the area beneath the ADU is enclosed. In many municipalities a certificate of no significant rise, or “no-rise” certificate is required when building in the floodway (this is usually in or adjacent to the active channel such as homes built on docks).
Seismic zones: In seismic and sloped hazard zones, foundations may require a geotechnical soils survey to specify foundation design standards. It is common for a grade-beam foundation to be required in order to comply with the higher structural requirement in these areas.
Tree Preservation: ADUs constructed near large trees such as redwoods or oaks must protect the root system of the tree during construction and foundation design must minimize excavations that could harm the root system. In these cases, a monolithic slab foundation is most commonly used.
Permits
Once you’ve selected a final design that works for your needs, your site, and meets regulatory standards, you can apply for the necessary permits which will vary by city. In most cases, a simple building permit will be enough to get your ADU construction started, and, if your project is built to the permitted standards, this will be followed with a certificate of occupancy. In some cases additional review is necessary when your ADU design requires a floodplain development permit or when design varies beyond what is allowed under the discretionary permit.
Construction
The construction process will often require signoff approval from the building department during construction stages for elements that will be hidden by future construction such as the foundation, utilities, and framing. Our Standard Materials Specs include:Site preparation including demolition if needed, and excavation
Excavation of your site for foundation and utility connections requires access for grading equipment and materials. This process is generally done at the beginning of the dry season and may require a stormwater control plan depending on the location and extent of the project. This is a messy part of the project and may require additional space for staging materials.
Foundation
The type of foundation required will depend on site conditions and your ADU design. Typical foundation types include monolithic slab, stem-wall, and grade beam.
Framing, Encasing, and Sub-flooring
Finishing