Garage conversions, attached and detached ADUs across San Francisco. Foundation, framing, plumbing, electrical, mechanical and finishes — coordinated through one contractor.
An accessory dwelling unit has its own kitchen, its own bathroom, its own entry and its own utilities. That means foundation, framing, plumbing, electrical, mechanical, insulation, drywall and finishes — the full sequence of building a house, compressed into a smaller footprint on a site that already has a building on it.
That last part is what makes ADUs harder than their size suggests. There is an existing house to work around, neighbours on both sides, limited access for materials and machinery, and utility runs that have to reach a new structure from a supply that was sized for one dwelling.
The structure, slab and roof already exist, which removes a large part of the cost. What replaces it is floor build-up, insulation, egress, ventilation, a full MEP fit-out and often structural work where the garage was carrying the house above.
Built off the existing house, sharing a wall and sometimes a foundation. Cheaper utility runs, but the connection to the existing structure has to be engineered and the existing building envelope has to be opened and made good.
A separate structure in the rear yard. Fewest constraints from the existing house, but everything is new — footings, framing, roof, envelope — and the utility runs are longer.
An ADU runs in a fixed sequence and skipping ahead costs money. Design and engineering come first, then permits, then site preparation and foundation, then framing, then the rough-in of plumbing, electrical and mechanical, then inspection, then insulation and drywall, then finishes, then final inspection.
Leafair takes on the construction from site preparation onward and coordinates the trades through that sequence. Design and structural engineering are handled by architects and engineers Leafair works with — you are told clearly which parts of the job sit with which party before anything is signed.
Whether a machine can reach the rear yard, or whether every yard of concrete and every stick of lumber is carried through the house by hand. This is one of the largest single variables on an SF lot.
Slope, soil and the depth required. A flat site with good soil and a shallow footing is a different job from a hillside needing retaining or piers.
How far water, sewer, gas and electrical have to travel to reach the new structure, and whether the existing service has capacity or needs upgrading.
Anything that touches the existing house — a shared wall, a garage that carries floors above — needs engineering and adds both cost and time.
A functional rental unit and a high-specification guest house use the same framing and very different kitchens, bathrooms, flooring and millwork.
What is found once things are opened. Older SF housing stock regularly hides deferred maintenance, undersized framing and dated electrical.
Narrow lots with no side access. Neighbouring structures within inches of the property line. Sloping rear yards. Sewer laterals that need replacing once they are exposed. Electrical services with no spare capacity. Foundations that were never designed to carry anything else.
None of these is unusual and all of them are manageable, but each one is a cost and a schedule item that should be identified before you commit, not discovered halfway through.
How to build an ADU in San Francisco walks the full process step by step. What drives ADU cost in San Francisco explains where the money goes. The San Francisco ADU permit process covers what happens between design and breaking ground.
Related Leafair services: foundations and structural work, home additions, and whole-home remodeling.
Most San Francisco lots with an existing home can accommodate an ADU in some form, but what is possible depends on the lot, the existing structure, access and utilities. The only reliable way to know is a site visit and a check against current planning and building requirements before any design work begins.
Design and engineering typically run for a period of months before construction starts, and the permit review adds time on top of that. Construction itself depends on whether the ADU is a garage conversion or new detached structure, on foundation work and on utility runs. Leafair gives you a schedule with the estimate rather than after work begins.
Usually, because the structure, slab and roof already exist. But garage conversions bring their own costs: raising floor levels, adding insulation and egress, upgrading electrical, and often structural work where the garage was carrying the house above. A detached build starts from nothing but has fewer unknowns.
An ADU needs permit drawings, and structural elements need engineering. Leafair is a licensed general contractor, not a design firm or an engineer, and will refer you to architects and engineers it works with regularly rather than pretending to cover those roles.
Leafair will walk the site, talk through what is realistic, and give you a written estimate priced line by line. Estimates are free.
