Redwood City · San Mateo County
ADU Builders in Redwood City: Architect-Led, Fixed-Price
SHAPE is a Mountain View design-build studio and licensed California general contractor (CSLB #1158688) building architect-designed modular ADUs across the Peninsula, including Redwood City. Fixed price after design, no budget surprises.
Building an ADU in Redwood City: Permits & Rules
ADU permits in Redwood City go through the Community Development Department, which handles planning review, building plan check, and inspections under one roof. Most single-family lots in the city qualify for a detached ADU, an attached ADU, or a garage conversion, and California state law makes that review ministerial, meaning it is approved on the code, not on neighborhood discretion.
A complete SHAPE submittal in Redwood City typically clears plan review in 8 to 14 weeks including corrections. State law requires the city to act within 60 days of a complete application, but real-world timelines depend on how clean the submittal is and how quickly corrections come back. We drive every cycle so the file keeps moving.
A few Redwood City rules worth knowing before you start:
- Setbacks. California state law caps side and rear setbacks at 4 feet for new detached ADUs, and Redwood City follows that standard. Front setbacks match the primary home's zoning.
- Height. Detached ADUs can be 16 feet tall, or 18 feet plus 2 feet for roof pitch within half a mile of a major transit stop. Attached ADUs can reach the zoning district's height limit.
- Owner occupancy. State law (AB 976) permanently bars cities from requiring the owner to live on a property with an ADU. You can rent both the primary home and the ADU, for stays of 30 days or longer.
- Utilities. Attached ADUs and JADUs usually piggyback on the primary home's water, sewer, and electrical service. Detached units sometimes require separate connections, especially when the panel is undersized or the sewer lateral needs to be upgraded. We confirm this on the site walk so it is priced in, not discovered mid-build.
- Parking. No replacement parking is required when a garage or carport is converted or torn down to build an ADU, wherever the lot is. Separately, no parking can be required for the ADU itself within one half-mile walking distance of public transit.
- Size. Under the city's ADU handout effective August 2026, detached ADUs can be 850 sq ft for a studio or one bedroom and 1,200 sq ft for two or more bedrooms. Attached ADUs can be up to 50% of the existing home, and zoning limits cannot prevent an 800 sq ft ADU.
- Units allowed. One detached ADU, one converted ADU and one junior ADU on a single-family lot. Junior ADUs are allowed in the R-1 and RH zones.
- Fire flow test. New detached ADUs need a fire flow test, which can take several weeks to schedule, so we order it early.
- Emerald Hills. Emerald Hills and Emerald Lake Hills are unincorporated San Mateo County, so they follow county rules, not Redwood City's. See our Emerald Lake Hills page.
Rules checked September 2026 against: Redwood City ADU handout, effective August 2026, Redwood City ADU page, Redwood City Master Fee Schedule, July 2026, Redwood City impact fees, July 2026, Redwood City tree permits, San Mateo County Assessor, HCD ADU Handbook, California Energy Commission ADU guidance. City rules change often. We confirm the rules for your lot before design.
Attached, Detached, or Garage Conversion?
Detached ADU. The strongest option for privacy, rental value, and long-term flexibility. It works especially well on the flatter lots in Friendly Acres, Roosevelt, and Woodside Plaza, where a modular set-and-finish can land a 500 to 900 sq ft unit without disturbing the main house. On Emerald Hills lots the same approach still works, but grading, retaining, and driveway access drive up site cost.
Attached ADU. A good fit when the main house has an underused wing or side yard that can be extended. Cheaper on utilities, since it usually shares service with the primary home, but it ties the two buildings together structurally and acoustically. On sloped Emerald Hills lots, an attached wing can save real money versus a fully independent detached foundation.
Garage conversion. The lowest-cost path in on most Redwood City lots, and typically the fastest permit. The tradeoff is fixed geometry, so you inherit the garage's dimensions, ceiling height, and existing slab. It shines on flat Friendly Acres and Roosevelt lots with a permitted, code-compliant garage already in place. It rarely pencils when the existing structure needs a new foundation or a full roof.
What an ADU Costs in Redwood City
Honest ranges, based on completed Redwood City projects in 2024 and 2025:
- Garage conversion. Typically from about $150K for a straightforward code-compliant conversion with modest finishes, up to roughly $250K when the slab, roof, or utilities need real work.
- Detached new-construction ADU. Typically $250K to $450K and up, depending on size (400 to 1,200 sq ft), finish level, and site conditions like grading, retaining, or a service upgrade.
- Attached addition ADU. Usually lands between the two, driven by how much of the existing structure has to be touched.
SHAPE quotes a guaranteed fixed all-in price after the design phase. That number covers architecture, engineering, permits, factory build, site work, utilities, and finishes. No allowances, no cost-plus, no mid-build surprises. If a site condition changes what has to be built, we say so up front instead of billing it as a change order.
City fees and capacity charges
These are Redwood City's own fees, separate from the building price. We include the ones your project needs in the fixed quote.
| Item | Amount |
|---|---|
| Base permit fee, up to 400 sq ft or garage conversion | $3,999.53 plus valuation-based fees |
| Base permit fee, 401 to 600 sq ft | $4,660.62 plus valuation-based fees |
| Base permit fee, 601 to 800 sq ft | $5,321.71 plus valuation-based fees |
| Transportation, parks and housing impact fees | $0 for ADUs of any size until April 1, 2027 |
| Wastewater treatment capacity fee | $1.78 per sq ft |
| Water capacity fee | $3.19 per sq ft |
| Separate address for the ADU | $376 |
| School fees | Apply to new space over 500 sq ft |
Trees, fire and flood checks in Redwood City
Most Redwood City lots are flat and simple, but a few site checks can still change the plan.
- Trees: private trees over 38 inches in circumference (12 inches in diameter) are protected. A removal permit has no fee and normally takes 10 working days.
- Fire: Redwood City's 2025 fire hazard map proposal showed no Very High parcels, down from 1,266 in 2007, with 60 High and 6 Moderate.
- Fire sprinklers: an ADU may need them if the main house already has them. An attached ADU may also need them if the addition and remodel reach 50% of the main home or 1,000 sq ft, whichever is less. Junior ADUs are exempt.
- Flood: in a FEMA flood zone, the ADU's lowest floor must sit above base flood elevation.
Sources for this section: Redwood City tree permits, Redwood City Pulse on the 2025 fire map, Redwood City ADU handout
Frequently asked
- Do I need a permit for an ADU in Redwood City?
- Yes. Every ADU in Redwood City requires a building permit and, for new construction, planning approval through the Community Development Department. Under California state law that review is ministerial, so it is approved based on code compliance, not neighborhood discretion. SHAPE runs the full permit package, corrections included.
- How long does the whole process take?
- For a typical Redwood City detached ADU, the build phase from permit to move-in is 2 to 4 months. Design and permitting typically add 8 to 12 weeks before that. We run those in parallel wherever the city allows, so garage conversions can move even faster.
- Can I rent out my ADU?
- Yes. State law allows long-term rentals of ADUs, and AB 976 permanently bars owner-occupancy requirements. Both the primary home and the ADU can be rented for stays of 30 days or longer. Redwood City prohibits rentals under 30 days for ADUs and junior ADUs.
- How big can my ADU be?
- Redwood City allows detached ADUs of 850 sq ft for a studio or one bedroom and 1,200 sq ft for two or more bedrooms. On top of local rules, state law guarantees at least 800 sq ft regardless of local floor-area limits. The right size for you depends on the lot, the primary house, and how you plan to use the unit.
- What makes modular different from site-built?
- In SHAPE's modular process, the structure is fabricated in a controlled factory environment while site work runs in parallel, then set and finished on your lot. That compresses the build schedule versus fully site-built construction, reduces weather risk, and holds the fixed price we quoted at design because the factory line does not slip.
- Does Redwood City have free or pre-reviewed ADU plans?
- Yes. The city hosts a pre-reviewed plans program that cuts city review to 30 days, and the county's free GLADUR plans come in a 400 sq ft studio, 575 and 745 sq ft one-bedrooms, and an 800 sq ft two-bedroom. They save design time but fit fewer lots than a custom plan. We can compare both on your lot.
- What is the ADU survey Redwood City asks for?
- The city asks every ADU homeowner to complete a short survey before the permit is finaled. It is part of the city's housing reporting, and we remind you before final inspection so it does not hold up sign-off.
- Can I legalize an older unpermitted unit in Redwood City?
- Often, yes. Redwood City's July 2026 code amendments were designed to help older ADUs get permits more easily, and state law (AB 2533) bars impact fees and penalties for ADUs built without permits before 2020, unless the unit is substantially unsafe.
- What is the difference between an ADU and a junior ADU?
- A junior ADU is up to 500 square feet inside the main house, plus up to 150 square feet for an entrance. It needs its own efficiency kitchen but can share a bathroom with the house. An ADU can be attached or detached, with its own full kitchen and bathroom.
- How many bedrooms can a Redwood City ADU have?
- State law does not let cities cap the number of bedrooms in an ADU. In Redwood City, two or more bedrooms raises the detached size limit from 850 to 1,200 square feet.
- Does a Redwood City ADU need solar panels?
- A new detached ADU does. Under the 2025 California Energy Code it needs new solar panels, which can be added to an existing system elsewhere on the property. An attached ADU or garage conversion counts as an addition or alteration and does not need new solar.
- Will an ADU raise my property taxes in Redwood City?
- Only on the new building. The San Mateo County Assessor does not reassess your existing home. It adds the ADU's market value at completion to your current assessed value, with a partial assessment if the work is only partly done on January 1.
- Can I sell the ADU separately?
- Not today. A city has to adopt its own ordinance under AB 1033 before ADUs can be sold separately as condominiums, and Redwood City has not.
- Can I build the ADU and a new main house at the same time?
- Yes. State law lets the city approve both together, and the ADU is still reviewed without discretionary review. When the two are built together, utilities can treat the ADU as new residential use for connection fees.
- Is the California $40,000 ADU grant still available?
- No. CalHFA's ADU Grant paid up to $40,000 toward predevelopment costs, and CalHFA says its latest round was fully allocated in December 2023. CalHFA warns that anyone offering to get you one is running a scam.
- How long does PG&E take to connect an ADU?
- PG&E reported ADU electric hookups averaging about 300 days from request for projects finished by August 2025, and says its average for new service had dropped to 118 days by March 2026. We file with PG&E when the permit goes in so power is not the last thing we wait on.
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