ADUs
Can I actually build an ADU on my lot?
California law made this question much easier to answer. What it did not do is make every lot the same.
For most single-family lots in Santa Clara and San Mateo counties, the answer is yes: state law has made ADUs broadly permissible. So the useful question is no longer am I allowed, it is where does it physically fit, and what will the utilities cost. Those two answers come from your rear-yard space after setbacks, access for equipment and utility runs, and whether your electrical service can carry another dwelling. That is a per-parcel answer, and it is worth getting before you pay anyone to draw anything.
Why “am I allowed” is usually the wrong question
California has spent several years removing local barriers to ADUs. The practical effect is that outright prohibition is now rare, and the constraints that decide your project are physical and financial rather than permissive.
Which is genuinely good news, because physical constraints can be measured. We would rather walk your lot for an hour and tell you the honest answer than have you spend $10,000 on plans for something the site cannot take.
The four things that actually decide it
Rear-yard space after setbacks
Not your total lot size, the usable rectangle left once setbacks are applied. This is the constraint that most often shrinks a project from what the owner pictured.
Access
How do materials, concrete and equipment reach the back of the property? A narrow side yard, a mature tree, a neighbor's fence line or a slope all add real cost. On some lots everything gets carried by hand, and that shows up in the number.
Utilities and distance
Water, sewer and electrical have to get there. The further the run and the more hardscape it crosses, the more it costs. Trenching through a finished driveway is not the same job as trenching through lawn.
Electrical service capacity
Adding a dwelling can push an older service past what it can carry. If it does, a service upgrade comes first, before anything else happens. This is the item that most often surprises people, and it is checked at the site visit rather than discovered at inspection.
Detached, attached, garage conversion or JADU
| Type | Needs | Usually chosen when |
|---|---|---|
| Detached | Rear-yard space, access, new foundation, utility runs | There is real yard to work with and privacy or rentability matters |
| Attached | Space alongside the house, new foundation, shorter utility runs | The yard is tight but there is room on a side or rear elevation |
| Garage conversion | An existing garage; usually slab remediation rather than a new foundation | The cheapest route to a real unit; the slab and the ceiling height are the usual sticking points |
| Junior ADU (JADU) | Space inside the existing house | Smallest scope and fastest path; no new footprint at all |
What it costs, and why we do not publish a per-square-foot number
A good-quality ADU in our area runs $300K–$360K for a good-quality build. We publish that as a project range and never as a rate per square foot, and the reason is arithmetic rather than coyness.
Roughly $13,500 to $18,500 of soft costs are fixed no matter how small the unit is: plans around $10,000, survey and certification about $3,000, and soils work from $4,500. A 400 sq ft unit therefore is not two-thirds the price of a 600 sq ft one. The effective rate per foot rises as the unit shrinks. Any contractor quoting ADUs off a flat multiplier has not priced one properly.
Since 1 January 2025 every California city and county must run a pre-approved ADU plan programme and must approve or deny a ministerial application using a pre-approved plan within 30 days. Where a plan fits your lot, most of the architectural set is already drawn and the city's decision shrinks to a 30-day clock; the site-specific sheets, utilities and construction keep their own schedule. It is worth asking before anyone starts drawing.
How to get a real answer in one visit
Bring whatever you have: a plot plan, an old survey, a photo of your electrical panel. We will look at the rear yard against setbacks, walk the access route, find where the utilities come in, and open the panel. That is normally enough to tell you which ADU type your lot actually supports and roughly what the site conditions will add.
If the answer is that your lot is a poor candidate, we would rather tell you on that visit than after you have paid for drawings.
Where to start, and what happens next
Most people who search for this are not asking a zoning question. They want to know what the first move is and who does what. Here is the whole path on an ADU built with us, in the order it actually happens.
Decide what the unit is for and name the parcel
Rental, a parent, an office, resale value later. The use drives the size, the kitchen and the entry, and it is the first thing we ask. Have the address ready so we can read the lot and the city rules before we talk.
Book a Project Discovery Call
A 20 to 30 minute video call. Bring whatever you have: a sketch, photos of the yard and the electrical panel, any plans, and your budget range. We tell you on that call whether the lot and the budget line up and what the site visit needs to settle.
Site visit
We measure the usable yard after setbacks, walk the utility route to the street, open the panel and look at access for equipment. If the lot is a poor candidate we say so here, before anyone pays for drawings.
Plans: your architect and engineer, coordinated by us
We are the builder, not the designer. We coordinate the architect and structural engineer who produce the stamped permit set with Title 24, or we start from a pre-approved plan where your city has one that fits. Either way the site-specific sheets still get drawn.
The proposal, walked line by line
Every scope line, allowance and exclusion in writing, reviewed with every decision-maker on a call before anything is signed. Site conditions, utilities and selections are named as the items that move the number.
Permits and the schedule
We submit to the city and carry plan-check corrections. The schedule is built on dependencies, not wishes: plan check, utility applications and material lead times set the start, and inspections set the milestones.
Foundation, envelope, trades, inspections
Foundation and framing, then roof and windows, then electrical, plumbing and mechanical rough-in, each with its city inspection before anything gets covered.
Finishes, final inspection, closeout
Drywall, floors, kitchen and bath, paint. The permit is finalled, you get the closeout package and the warranty terms in writing.
What you receive along the way
A daily log with photos from the site, the live schedule, invoices tied to milestones and change orders in writing before the work, all in one client portal.
The pages that go with this: what we build and what an ADU costs, the San Jose cost breakdown, how long an ADU takes, garage conversions, pre-approved plans and AB 1332 and finished projects. In San Jose the city's own ADU page is the official source for its process and pre-approved plans: sanjoseca.gov, Accessory Dwelling Units. Every other city we serve publishes its own, and we read the right one for your parcel.
Book a Project Discovery Call with your address and your budget range. We serve Santa Clara and San Mateo counties.
Common questions
How much yard do I need for a detached ADU?
Do I need a separate electrical service for an ADU?
Can I rent it out?
Is a garage conversion cheaper than building detached?
Do you design the ADU?
Where to go next
Written by the team at Elusive Construction, a licensed California general contractor (B #1112902) building in Santa Clara and San Mateo counties. General information about how residential construction works, not legal advice. For your specific contract, talk to an attorney.
Also Worth Reading
The rest of what we publish
Cost guides
What an ADU, an addition, a kitchen or a whole house actually costs here, and what moves each number.
Our own projects
Our projects written up start to finish, including what we found once the walls were open.
Paying for it
Renovation lenders we work with. We take no referral fee, and we never quote you a rate.
Where we build
Sixteen cities across two counties, and what genuinely changes at each city line.
Have a project you want priced honestly?
Book a Project Discovery Call and we'll tell you what's realistic on scope, budget and timeline, even if we're not the right builder for it.