Converting a San Francisco Basement or Lower Level Into an ADU
San Francisco builds most of its ADUs inside buildings it already has — in Victorian and Edwardian ground floors, and in the garage-and-storage levels under Sunset and Richmond rows. Whether yours works comes down to four gates, and the first one is a tape measure.
The short answer
Four gates decide a lower-level conversion: ceiling, egress, structure, services
- Ceiling: measure the finished ceiling after new floor build-up. If you are short, the choices are dig down (underpinning), re-scope, or stop.
- Egress: every sleeping space needs a code-compliant emergency escape opening, with light and ventilation.
- Structure: underpinning and foundations decide whether digging down is viable — especially in soft-story buildings.
- Services: if tenants use the space for parking, laundry or storage, the housing-services check on the rent-control page is mandatory before you design anything.
Planning range for this work: $120K–$250K — planning range (secondary sources, checked Nov 2026) — not a quote, not permit data. Where you land inside it is decided by those four gates, not by the size of the room alone.
Sources: planning range (secondary sources, checked Nov 2026) — published SF cost guides including ADURegulations.org SF cost rows and a contractor cost guide whose $150–220K basement row sits inside the range. San Francisco permit context from SF open permit data (DBI via DataSF) — see the permit-data section. State programme conversion allowance attributed to the ADURegulations.org summary of §207.2; SF Planning confirms for a specific property. The screening numbers on this page are DBI's own: SF Department of Building Inspection, ADU Design & Permitting Resource Sheet, updated October 2025 — confirm with DBI how they apply to your building. DBI's ADU Intake Screening Checklist (sf.gov) is the second named DBI source for the same screening thresholds; the numbers published on this page are the Resource Sheet's, and DBI confirms which document governs your building.
Why conversions lead in San Francisco
SF builds ADUs by converting first
SF open permit data (DBI, DataSF) shows SF ADU production is conversion-dominant: of 469 ADU-flagged permits filed 2023–2026, conversions and garage conversions are the largest classifiable types and detached new construction is the minority. That matches the stock: deep Victorian and Edwardian ground floors, rear-yard-facing lower levels, and the garage-and-storage levels under Sunset and Richmond stucco rows.
Two limits on that data, stated plainly. SF publishes no per-permit fees in open data, so no fee claims can be drawn from it. And declared (estimated) costs in that data are applicant estimates at filing — they are not used here as a cost range and do not validate the planning range below.
Illustration only. A cyanotype-style cross-section of a Victorian ground-floor conversion, drawn to show the relationship between the house above and the new unit below. It is an illustration, not a design for any property.
Gate 1
Ceiling height — measure the finished ceiling, not the room you have
The measurement that matters is the finished ceiling: after the new floor build-up — structure, insulation, finishes — has taken its share of the height you measured on a site visit. A lower level that feels tall enough standing in it today can finish short once the floor is built.
Measure first, before design money. If the finished height is short, there are only three honest choices: dig down (lower the floor, which means underpinning — Gate 3), re-scope the unit to what the level can lawfully hold, or stop. There is no fourth choice in a waiver: on the sf.gov Local Program rules, planning waivers are never available for ceiling height — see the rent-control and programme-choice guide.
The numbers DBI publishes. DBI's own ADU Design & Permitting Resource Sheet (updated October 2025) sets the screening numbers: finished ceilings of 7 ft 6 in in living/sleeping areas and hallways, 7 ft in kitchens, bathrooms, storage and laundry. Below those finished heights, the choices are dig down, re-scope, or stop. Measure after the new floor build-up, not today's clear height. The same sheet sets the unit minimum at 220 sq ft, habitable rooms at least 70 sq ft and at least 7 ft in any dimension, with at least one room of more than 120 sq ft. The full measure-it-yourself checklist is below.
Just changed: Ordinance No. 234-26 (Board File 260623), approved by the Mayor on Nov 4, 2026, lowers San Francisco's Housing Code minimum ceiling height in existing buildings to 7 ft for habitable rooms and 6 ft 8 in for other rooms. Ordinances normally take effect 30 days after enactment — confirm the effective date, and how DBI applies it alongside the Building Code and the state ADU standards, before designing to it. DBI's published screening sheet still shows 7 ft 6 in / 7 ft until it is updated. (Board of Supervisors legislative record, read Nov 5, 2026.) The resource sheet is a screening guide, not the whole code: DBI confirms what applies to your building.
Gate 2
Egress, light and ventilation
Every sleeping space needs a code-compliant emergency escape opening — a way out that works when the hallway does not — together with adequate light and ventilation for a habitable room. In a lower level, that usually means creating or enlarging an opening to the rear yard or a lightwell, and designing the bedroom around it rather than placing the bedroom first and hunting for egress later.
One allowance helps here. Under the State programme conversion allowance — secondary attribution: ADURegulations.org summary of §207.2 — conversions within the existing structure carry no size cap, and up to 150 sq ft may be added for ingress/egress. Attribute it exactly that way: it is a summary of the state programme, and SF Planning confirms for a specific property whether and how it applies to yours.
DBI's resource sheet (updated October 2025) puts the egress, light and ventilation numbers in one place: the emergency escape opening in every bedroom/sleeping area must be at least 5.7 sq ft, at least 24 in high and 20 in wide, with its sill less than 44 in above the floor, opening freely without keys or tools. Most ADUs require two exits; the exit path must be 36 in wide, every door needs a 32-in clear opening (usually a 34-in door panel), and the farthest point in the unit to the public way must be less than 125 ft. Habitable rooms need glazing of at least 8% of the floor area, and openable ventilation of at least 4% of the floor area (an exterior door also counts; mechanical ventilation if that is not possible). Confirm with DBI how each number applies to your building.
Measure it yourself
The DBI self-screening checklist
Every number below is from one source: SF Department of Building Inspection, ADU Design & Permitting Resource Sheet, updated October 2025. Measure your lower level against it before you pay for design work.
| Check | DBI screening number |
|---|---|
| Finished ceiling — living, sleeping, hallways | 7 ft 6 in |
| Finished ceiling — kitchen, bath, storage, laundry | 7 ft |
| Emergency escape opening (every bedroom/sleeping area) | At least 5.7 sq ft, at least 24 in high, at least 20 in wide, sill less than 44 in above the floor; opens freely without keys or tools |
| Exits | Two exits for most ADUs; exit path 36 in wide; door clear opening 32 in; travel distance to the public way less than 125 ft |
| Unit size | At least 220 sq ft |
| Habitable rooms | At least 70 sq ft and at least 7 ft in any dimension; at least one room more than 120 sq ft |
| Light (glazing) | At least 8% of the floor area of the habitable room |
| Ventilation (openable) | At least 4% of the floor area (an exterior door also counts; mechanical ventilation if not possible) |
One nuance: state-law ADUs may not have to follow all of these requirements — the 7 ft 6 in figure is the Housing Code / local standard, and a state-programme (§207.2) conversion is confirmed against state building standards by DBI. If you are filing under the state programme, confirm which standard DBI applies to your building.
Just changed: Ordinance No. 234-26 (Board File 260623, approved Nov 4, 2026) lowers the Housing Code minimums in existing buildings to 7 ft (habitable rooms) and 6 ft 8 in (other rooms). The figures in the table above are DBI's published sheet, which predates it — confirm with DBI which figure governs your permit, and from what date, before designing a 7 ft 0 in – 7 ft 5 in level. (Checked Nov 5, 2026.)
Gate 3
Structure and underpinning — where the budget is really decided
Digging down is foundation work. The cost drivers are the ones under the floor: foundation type and depth, soil, and shared walls with neighbouring buildings. Two lower levels with identical floor plans can price very differently because one sits on a foundation that can be underpinned straightforwardly and the other shares walls and shallow footings with the houses on both sides.
Soft-story buildings: pair the projects, or plan around both
Many SF ground floors sit in soft-story buildings — a flexible, open ground floor (garages, storage) under stiffer living floors above. Ground-floor conversion work in a soft-story building interacts with seismic retrofit: you are working in exactly the storey the retrofit strengthens. Pairing the conversion with the retrofit is common SF practice, and it is usually better planned as one coordinated project than discovered as a conflict mid-permit. If your building is soft-story, raise the pairing question at the first structural conversation, not the last.
Fire safety
Fire separation, sprinklers and fire-flow
Fire separation between the new unit and the house above is a standard requirement for a lower-level conversion — the new home below and the home above must be properly separated. Whether sprinklers or fire-flow requirements are also triggered depends on the building. Those triggers are confirmed by DBI for your property; this page states no invented thresholds, and no waiver route changes the position on fire safety or exits — waivers are never available for those items.
Gate 4 — mandatory
The housing-services check comes before the cost conversation
If any tenant in the building uses your lower level — for a garage, parking, laundry, storage, a deck or a garden — that use is a housing service under the Rent Ordinance, and removing or reducing it carries serious legal risk. The full explanation, including why building an ADU is not by itself just cause, lives on the rent-control page — read it before you commit to a design that takes tenant space. No ceiling measurement or cost estimate outranks this check.
Costs
Where the planning range comes from — and what moves you inside it
That label applies to every appearance of the figure on this page: $120K–$250K is a planning range (secondary sources, checked Nov 2026) — not a quote, not permit data. Inside the range, underpinning is the biggest mover, followed by how much egress work the level needs, the fire-separation and any sprinkler scope DBI confirms, and how much of the level is already finished, plumbed and heated. A level that needs no dig-down and already has compliant egress sits at a very different point from one that needs both.
Basement conversion vs garage conversion
| Basement / lower-level conversion | Garage conversion (comparison only) | |
|---|---|---|
| Starting space | Basement, storage level or ground floor under the main house | Existing garage, usually at the ground-floor front or rear |
| Deciding gate | Finished ceiling height; dig-down if short | Headroom is rarely the gate; frontage, light and the loss of parking usually are |
| Typical SF stock | Victorian / Edwardian ground floors; rear-yard-facing lower levels | Sunset / Richmond garage-and-storage levels; ground-floor garages citywide |
| Services risk | Storage and laundry commonly serve tenants — check first | Parking commonly serves tenants — check first |
The comparison is here to help you name your space correctly — this page targets lower-level conversions, and the services risk applies to both rows. For what fits a garage level on a 25-foot lot in District 4 specifically, see the Sunset District guide.
Questions, answered
Before you measure anything else
Is my ceiling high enough — and what if it finishes short?
Measure the finished ceiling after new floor build-up, not today's clear height. If it is short, the choices are dig down (underpinning), re-scope, or stop. There is no waiver for ceiling height. DBI's ADU resource sheet (updated October 2025) sets finished ceilings at 7 ft 6 in in living/sleeping areas and hallways and 7 ft in kitchens, bathrooms, storage and laundry; Ordinance No. 234-26 (Board File 260623, approved by the Mayor Nov 4, 2026) lowers the Housing Code minimum in existing buildings to 7 ft for habitable rooms and 6 ft 8 in for other rooms — confirm the effective date and which figure DBI applies to your permit before designing to it.
What egress does a basement bedroom need?
Every sleeping space needs an emergency escape opening — DBI's ADU resource sheet (updated October 2025) lists at least 5.7 sq ft, at least 24 in high and 20 in wide, sill less than 44 in above the floor — with glazing of at least 8% of the floor area and openable ventilation of at least 4%. Under the State programme summary (§207.2, per ADURegulations.org), conversions within the existing structure carry no size cap and up to 150 sq ft may be added for ingress/egress — SF Planning confirms for a specific property.
When is underpinning required, and what drives its cost?
When the finished ceiling only works by lowering the floor. Cost turns on foundation type and depth, soil, and shared walls — and, in a soft-story building, on coordinating with seismic retrofit work in the same storey.
Will sprinklers be triggered?
Fire separation between the new unit and the house above is a standard requirement. Sprinkler and fire-flow triggers depend on the building and are confirmed by DBI — there is no honest generic threshold, and fire safety and exits can never be waived.
Where does my project land in the $120K–$250K range?
The range is a planning range (secondary sources, checked Nov 2026) — not a quote, not permit data. Underpinning, egress work, fire scope and how finished the level already is decide your point inside it. A builder's site visit produces the real number.
My tenants store things in the basement. Can I still convert it?
Storage is a housing service. Read the rent-control guide's housing-services section and confirm your position with the Rent Board or counsel before designing around that space — building an ADU is not, by itself, just cause to remove a service.
San Francisco guides
Related guides
Will your ADU be rent controlled?
The programme-choice decision table and the mandatory housing-services check.
San FranciscoTwo detached ADUs from 2027
What AB 956 changes — and why most conversion lots will still convert first.
San FranciscoSell an ADU separately — AB 1033
The eligibility test for selling a converted unit as a condo.
Check your lower level
The ceiling question comes first
The checker is pre-set to San Francisco with “Basement conversion” selected. Tell us the address and what the level is used for today — the ceiling question comes first in your plan, before any cost conversation, exactly as it should on site.
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