What drives the cost of building an ADU in San Francisco — site access, excavation, foundation, utility runs, structural requirements and finish level, explained.
There is no reliable per-square-foot figure for a San Francisco ADU. Cost is driven by site access, excavation and foundation, how far utilities must run, whether the existing structure needs structural work, and the finish level chosen. Two identical-sized ADUs on two SF lots can differ substantially on those factors alone.
Leafair does not publish an ADU price range, because a range wide enough to be honest is too wide to be useful, and a range narrow enough to be useful would not be honest.
What is useful is knowing which decisions move the number and by roughly how much, so that when you do get an estimate you can read it rather than just compare totals. That is what this guide covers.
| Cost driver | What it depends on |
|---|---|
| Size and layout | Total floor area, and whether the plan needs one bathroom or two, one wall of kitchen or a full run. |
| Site access | Whether machinery can reach the work. Hand-digging and carrying spoil through a house is slow, and slow is expensive. |
| Excavation | Depth required, slope of the lot, soil conditions and whether shoring or retaining is needed. |
| Foundation | Conventional footings on a flat site versus stepped footings, grade beams or piers on a slope. |
| Utility runs | Distance from the existing supply to the new structure, and whether the existing water, sewer, gas or electrical service has spare capacity or needs upgrading. |
| Structural requirements | Anything touching the existing house — a shared wall, a garage carrying floors above — needs engineering and adds both cost and programme. |
| Kitchen | Cabinet run length, worktop material, appliance specification. The single most variable room per square foot. |
| Bathroom | Number of bathrooms, whether a shower pan is formed on site, tile selection and glass. |
| Finish level | The same framing can be finished as a functional rental or a high-specification guest house. This is the largest discretionary variable in the whole budget. |
| Engineering and permits | Structural engineering fees and permit costs, both outside a contractor’s control and both real line items. |
| Existing conditions | What is found when things are opened. Deferred maintenance, undersized framing and dated electrical are common in older San Francisco housing stock. |
Every trade as its own line — excavation, foundation, framing, roofing, windows, plumbing, electrical, mechanical, insulation, drywall, kitchen, bathroom, flooring, paint — with overhead and profit shown separately rather than buried.
Allowances stated openly where a selection has not been made, so you know which numbers are fixed and which will move. And a written process for how changes get priced once work starts, because on a project of this length there will be changes.
How to build an ADU in San Francisco covers the twelve stages in order. The San Francisco ADU permit process covers what happens before construction. For a written estimate on your property, see ADU construction with Leafair.
Because it hides the things that actually decide the cost. Two ADUs of identical size on two San Francisco lots can differ enormously depending on access, foundation, utility runs and what the existing structure demands. A per-square-foot figure quoted before a site visit is a guess with a decimal point on it.
Utilities and access. Getting water, sewer, gas and power to a new structure at the back of a lot, and getting materials and machinery in to build it, are where budgets move most on constrained San Francisco properties.
Usually, because the structure already exists — but not always. Raising the floor, adding egress and ventilation, upgrading electrical and dealing with a garage that carries the house above can close the gap quickly.
Leafair will walk the site, talk through what is realistic, and give you a written estimate priced line by line. Estimates are free.
