Quick Answer: An ADU in San Mateo costs $395 to $610 per square foot all-in, so a 750 square foot detached unit runs $296,000 to $458,000 and a garage conversion $165,000 to $285,000. State law gives the city 60 days to approve or deny a complete application, which is a genuine protection and the main reason an ADU permits faster than an addition here. On San Mateo’s narrow lots the constraint that decides feasibility is usually not the rules, it is where the sewer lateral runs and whether equipment can reach the back garden.
San Mateo is a good ADU market and a physically awkward one. The lots are often 40 to 50 feet wide with a driveway down one side, the houses are frequently 1920s to 1950s on raised foundations, and the back gardens are reachable only through a side yard that may be five feet clear.
State ADU law has removed most of the regulatory obstacles. What remains is site logistics, and that is what separates a project that pencils from one that does not.
What it costs
| Type | Per square foot, all-in | Typical project |
|---|---|---|
| Garage conversion, existing footprint | $330 – $475 | $165,000 – $285,000 for 400 – 600 sq ft |
| Junior ADU inside the house | $300 – $430 | $105,000 – $215,000 for 350 – 500 sq ft |
| Attached ADU, new construction | $385 – $560 | $231,000 – $392,000 for 600 – 700 sq ft |
| Detached ADU, 750 sq ft | $395 – $610 | $296,000 – $458,000 |
| Detached two-storey or above-garage ADU | $450 – $640 | $360,000 – $512,000 for 800 sq ft |
All-in covers design, engineering, city and utility fees, construction and finishes. It is worth insisting on this basis when comparing builders, because a construction-only number looks 25 to 30 percent lower.
What the rules allow
| Item | Generally |
|---|---|
| Maximum size | Typically 850 sq ft for a studio or one bedroom, 1,000 for two or more |
| Setbacks, detached new construction | Four feet from side and rear property lines |
| Conversion of an existing structure | Existing setbacks may generally be retained |
| Parking | Not required for an ADU within half a mile of transit, which covers much of San Mateo |
| Owner occupancy | Not required for an ADU under current state law |
| Review clock | 60 days for a complete application |
| Design review discretion | Limited by state standards, unlike an addition |
| Impact fees | Exempt below 750 sq ft |
| School fees | Apply above 500 sq ft only |
Rules change, so confirm the current standards for your parcel before design. The two thresholds worth deciding deliberately are 500 square feet, below which school fees do not apply, and 750 square feet, below which impact fees do not apply. That second one is why so many ADUs are built at 749 square feet.
The constraints that actually decide feasibility in San Mateo
1 · Where the sewer lateral runs, and its condition
The most consequential unknown and the cheapest to investigate.
| Item | Cost |
|---|---|
| Camera inspection and locate | $400 – $1,200 |
| New lateral run from the ADU to the main | $6,000 – $22,000 |
| Lateral replacement, where the existing one is clay or failing | $12,000 – $38,000 |
| Sewer capacity or connection fee | $2,500 – $12,000 |
In pre-1955 San Mateo homes the lateral is frequently vitrified clay with root intrusion. The city may require its replacement before allowing a second connection, and that is a $12,000 to $38,000 decision made for you. Camera it before you choose where the ADU sits: a $700 inspection can reposition the unit and save $20,000.
2 · Whether equipment can reach the back garden
This is the San Mateo-specific one, and it is frequently discovered late.
| Side yard clear width | What is possible | Cost impact |
|---|---|---|
| 10 ft or more | Small excavator, direct concrete truck access | Baseline |
| 6 – 9 ft | Mini excavator, concrete pump | +$6,000 – $16,000 |
| 4 – 6 ft | Hand excavation, pumped concrete, hand-carried materials | +$18,000 – $45,000 |
| Under 4 ft, or no side access | Crane over the house, or removing fencing and crossing a neighbour’s land by agreement | +$25,000 – $70,000 |
On a 40-foot San Mateo lot with a house 30 feet wide, the clear side yard after eaves and a gas meter can be under five feet. That does not stop the project, but it changes the number materially and it should be priced at the start rather than discovered at excavation.
It also affects the choice between a site-built and a prefabricated unit. A prefab module cannot be craned in if there is no crane position or overhead clearance, and on many San Mateo lots there is not.
3 · Electrical service capacity
The load calculation covers the existing house plus the new unit. Many pre-1955 San Mateo homes are on 100 amps or less, sometimes with knob and tube wiring still in place.
- 200-amp service upgrade: $5,000 to $11,000
- Separate meter for the ADU: $3,500 to $9,000, and worth considering if you intend to rent
- Undergrounding an overhead service drop, sometimes required: $8,000 to $25,000
- Partial rewire of the main house, where knob and tube is present in circuits affected by the work: $8,000 to $26,000
Run the load calculation before design. It is the most common source of a correction at plan check and it changes the panel and meter drawings.
4 · Trees and the raised foundation
Mature trees are common on these lots and protected trees restrict where you can build. A tree survey at $800 to $3,000 establishes the buildable envelope before an architect works inside a false one.
Separately, where the ADU attaches to the main house or sits close to it, the existing raised foundation gets exposed and any settlement or dry rot becomes visible and, generally, becomes work.
How long it takes
| Stage | Duration |
|---|---|
| Site investigation: sewer camera, load calculation, tree survey, access assessment | 2 – 4 weeks |
| Design and engineering | 8 – 16 weeks |
| Plan check under the 60-day rule, including one correction cycle | 8 – 14 weeks |
| Permit issuance | 1 – 3 weeks |
| To permit in hand | 4 – 8 months |
| Construction, garage conversion | 4 – 7 months |
| Construction, detached unit | 8 – 13 months |
The 60-day clock only starts on a complete application. A submittal missing the Title 24 report, the structural calculations or the electrical load calculation is returned, and the clock restarts on resubmittal. Every incomplete submittal costs a month, and it is the one delay entirely within your control.
How to choose a builder here
Five questions that separate builders who know San Mateo from builders who do not:
- Have you cameraed the sewer lateral, and what did it show? A builder who has not is guessing at a $38,000 line item.
- What is the clear side yard width, and how does your price account for access? A number that assumes truck access on a five-foot side yard is wrong by $20,000 or more.
- Has the electrical load calculation been run for house plus ADU? This is the most common plan check correction.
- Is the price all-in, including design, engineering, city and utility fees? Ask for those as separate lines.
- What is the plan if plan check returns corrections, and who pays for the revisions? A good answer is specific.
Also run the ordinary checks: active state licence with the right classification, current workers’ compensation, general liability with a certificate sent by the insurer directly, and references from projects finished 18 to 36 months ago rather than last month.
Does it pay?
San Mateo rents support ADUs well, and a detached one to two bedroom unit here rents at the upper end of the Peninsula range. Whether it pencils depends on the site costs above rather than the rent, which is the point of investigating them first.
Two things to note. New square footage is reassessed for property tax at market value and added to your base, permanently, while the rest of the house keeps its protected Proposition 13 basis. And an ADU generally adds to property value, though appraisers treat it inconsistently, so do not assume the value added equals the cost.
The right first step
Before design, spend a few thousand dollars establishing four facts: what the sewer lateral is made of and where it runs, what the electrical service can carry, what the clear side yard access allows, and whether any protected tree limits the envelope. Those four determine both feasibility and price, and every expensive surprise on a San Mateo ADU traces back to skipping them.
King David Home Builders designs and builds ADUs throughout San Mateo, Palo Alto, Redwood City and the Peninsula, and runs the sewer, electrical, tree and access investigation before drawing anything. See our ADU builders in San Mateo page for project detail and references.
Frequently Asked Questions
How much does an ADU cost in San Mateo?
$395 to $610 per square foot all-in, so a 750 square foot detached unit runs $296,000 to $458,000. A garage conversion is $165,000 to $285,000, a junior ADU inside the house $105,000 to $215,000, and an attached new-build ADU $231,000 to $392,000. All-in includes design, engineering, city and utility fees, construction and finishes.
What decides whether an ADU is feasible on a San Mateo lot?
Usually not the rules but the site logistics. Two things dominate: where the sewer lateral runs and what it is made of, since a failing clay lateral may have to be replaced at $12,000 to $38,000 before a second connection is allowed; and whether equipment can reach the back garden, since a clear side yard under six feet adds $18,000 to $45,000 and under four feet can require a crane over the house.
How long does an ADU permit take in San Mateo?
Four to eight months to permit in hand, then four to seven months of construction for a garage conversion or eight to thirteen for a detached unit. State law requires the city to approve or deny a complete application within 60 days, but that clock only starts when the application is complete, so an incomplete first submittal effectively costs a month.
Why are so many ADUs built at 749 square feet?
Because units below 750 square feet are exempt from impact fees that larger units pay. There is a second threshold at 500 square feet, below which school impact fees do not apply at all. If your plan is near either number, make the decision deliberately rather than by accident.
Can I use a prefabricated ADU in San Mateo?
Sometimes, and site access is what decides it. A prefab module has to be craned into position, which needs a crane setup location and overhead clearance. On many San Mateo lots, 40 to 50 feet wide with mature trees and overhead utilities, neither exists. Establish access feasibility before committing to a prefab approach, because it is not a decision you can reverse cheaply.