Building an ADU in San Francisco: 2025 Rules & Costs
- Gilad Shamri
- Sep 17
- 3 min read
Your guide to building an ADU in SF — updated rules, costs, and timelines
What is an ADU?
An Accessory Dwelling Unit (ADU) is a self-contained residential unit located on the same lot as a single-family home or multifamily property. In San Francisco, ADUs are commonly known as “in-law units,” “granny flats,” or “secondary units.”
An ADU must have its own kitchen, bathroom, and living area, making it suitable as a rental property or independent living space. There are two main types in San Francisco:
Attached ADUs – Converted from existing space (basements, attics, or garages) or built as an addition to the home.
Detached ADUs – Standalone backyard cottages or units built separately from the main home.
2025 Updates to ADU Rules in San Francisco
San Francisco has actively supported ADU development as part of its housing strategy. In 2025, several important changes aim to simplify the process and encourage more homeowners to build.
1. Streamlined Permitting
The Department of Building Inspection (DBI) has reduced approval times for qualifying ADUs. Most projects that meet zoning and building standards can expect permits in 6–9 months, compared to the 12+ months of past years.
For single-family properties, many ADUs now qualify for ministerial approval, meaning they avoid lengthy discretionary reviews.
2. Parking Requirements
No parking is required for new ADUs if the property is within half a mile of public transit—covering most of San Francisco.
3. Rent Control & Tenant Protections
ADUs built after 1979 remain exempt from traditional rent control laws, but they must comply with San Francisco’s tenant protections.
4. Utility & Energy Standards
Starting in 2025, new ADUs must meet California’s updated energy codes, requiring electric-ready systems, efficient insulation, and solar-readiness.
Gas hookups are increasingly restricted, pushing most ADUs toward all-electric design.
5. Size & Design Standards
Detached ADUs can now be up to 1,200 square feet, depending on lot size.
Height limits remain 16–20 feet for most detached ADUs.
Setbacks have been reduced, allowing homeowners to build closer to property lines.
How Much Does It Cost to Build an ADU in San Francisco in 2025?
Costs for building an ADU in San Francisco depend heavily on the type of unit, site conditions, and level of finishes. Below is a general breakdown:
Garage or Basement Conversion:
Low-end: $175,000 – $225,000
Average: $225,000 – $300,000
High-end: $350,000+
Detached Backyard Cottage:
Low-end: $300,000 – $400,000
Average: $400,000 – $550,000
High-end: $600,000+
Cost Per Square Foot:
On average: $500–$700 per sq. ft
Luxury builds or difficult sites can exceed $800 per sq. ft.Soft Costs (20–30% of total budget):
Architectural & engineering fees: $20,000 – $50,000
Permits & city fees: $15,000 – $30,000
Impact fees (waived for smaller ADUs but may apply to larger builds)
Timelines: How Long Does It Take to Build an ADU?
A realistic timeline for building an ADU in San Francisco in 2025 looks like this:
1. Design & Feasibility (2–3 months) – Site surveys, architectural drawings, and initial budgeting.
2. Permitting (6–9 months) – Faster than in previous years, though historic or unusual lots may take longer.
3. Construction (6–12 months)
Conversions: typically 6–8 months
Detached builds: 8–12 months
Total timeline: 12–18 months from start to finish.
Financing Options for ADUs in San Francisco
Building an ADU is a major investment, but homeowners now have more ways to finance projects:
Home Equity Line of Credit (HELOC)
Cash-Out Refinance
ADU-Specific Loan Programs
City & State Incentives (e.g., CalHFA’s ADU Grant Program, when available)
Common Challenges & How to Overcome Them
1. Space Limitations: Many San Francisco lots are small or sloped. Work with an architect experienced in maximizing space.
2. Construction Costs: Inflation and labor shortages keep costs high. Request multiple bids and consider design-build firms.
3. Historic District Restrictions: Expect additional review and design requirements.
4. Neighbor Concerns: Communicate early with neighbors to avoid disputes.
Why Build an ADU in 2025?
Rental Income: A one-bedroom ADU can bring in $2,500–$3,500 per month. Property Value: Adding an ADU often increases property value by 20–30%.
Flexibility: Ideal for family housing, home office, or guest suite.
Future-Proofing: Long-term value as housing needs evolve.
Building an ADU in San Francisco is still a complex process, but the 2025 updates make it more accessible than ever for homeowners. With streamlined approvals, flexible design standards, and strong rental demand, an ADU can be one of the smartest investments. Consult with an architect or design-build firm familiar with San Francisco’s codes to set your project up for success.
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