ADU Builder in Belmont
Belmont is a hillside city, and on a hillside the slope decides your ADU before the architect does. Here is how Belmont's own code actually works, including the two rules that catch almost every owner out.
On a Belmont slope, the ground writes the brief
Belmont's own zoning purpose clause says the quiet part out loud: the Council found geologic and soils hazards, steep slopes and inadequate roadways, and set out to reduce density as slope increases. That philosophy runs through every number below.
Slope decides your floor area, not your lot size
In the hillside districts, allowable floor area is set by a sliding scale against average lot slope. It runs from 0.350 on gentle ground down to 0.026 at 45% and steeper. On a steep lot the existing house can consume the district's entire allowance, leaving nothing for an ADU on paper.
The 800 sq ft rule is what makes it buildable
Section 24 lets you build an ADU of up to 800 sq ft even where that pushes the site over its floor area ratio, and the site is then treated as legally non-conforming. On a steep Belmont lot that allowance is frequently the only thing making an ADU possible, and it means your realistic ceiling is 800 sq ft rather than the 850 or 1,000 the code otherwise allows.
Height is measured per cross-slope section, with no averaging
Belmont measures from finished grade at each point around the perimeter to the top of the roof, for each cross-slope section. There is no average-grade method to fall back on. On sloping ground that is materially stricter than it sounds, and it is why a design that works on a flat lot often does not work here.
An adopted geologic hazard overlay from 1985
Belmont adopted the Cotton engineering geologic and ground movement potential maps for the San Juan Hills area, and they still govern. Parcels are rated, and the ratings carry real force: an actively moving deep landslide or potential debris flow parcel is simply not permitted for residential use until the map itself is changed.
The trap: an ADU is living space
Belmont lets accessory structures such as garages, decks and gazebos be permitted on hazard-rated parcels without extra geologic investigation. That exception is expressly limited to structures not intended for living space. An ADU is living space, so it does not qualify. On a mapped parcel your ADU goes the full city geologist route, and a project deviating from the criteria table goes to the Planning Commission.
Belmont's Safety Element background report states plainly that above 30% slope conventional single-pad construction is unsuitable and building requires substantial grading and retaining walls. If your lot is in that band, budget for the ground before you budget for the finishes.
What Belmont allows
Belmont Zoning Ordinance Section 24. Worth knowing before you rely on it: Section 24 has not been amended since August 2020, while the sections around it were updated through December 2025, and it still cites California code sections that have since been renumbered. Where it conflicts with current state law, state law governs.
| Unit type | Maximum size | Height | Setbacks |
|---|---|---|---|
| Junior ADU (JADU) | 500 sq ft, within the existing house | n/a | n/a |
| Attached ADU | 850 sq ft, or 1,000 sq ft with 2+ bedrooms, and no more than 50% of the existing home's living area | Per the underlying district | Per the underlying district |
| Detached ADU, single story | 850 sq ft, or 1,000 sq ft with 2+ bedrooms | 16 ft | 4 ft side and rear, 5 ft from the main house |
| Detached over a garage, flat roof | As above | 20 ft, sited 4 ft from the rear or side line | 4 ft side and rear |
| Detached over a garage, pitched roof | As above | 23 ft, sited 4 ft from the rear or side line | 4 ft side and rear |
| Detached over a garage, full setbacks | As above | 28 ft where it meets the district's own side and rear setbacks | District standard |
- The height ladder is the unusual part. Most cities give you one number; Belmont gives you four, and which one you get turns on whether the unit sits over a garage, whether the roof is flat or pitched, and whether you take the 4 ft setback or the district's full setback.
- No parking is required for an ADU or JADU. None at all.
- Minimum size is the state efficiency-unit minimum, currently 150 sq ft. There is no minimum lot size, and an ADU may go on a lot that does not meet the district's dimensional requirements.
- Interior ADUs and JADUs are exempt from the underlying district's setback, height and floor area standards entirely.
- Belmont's code contains an owner-occupancy clause written to resume in January 2025. Current state law does not permit that for an ADU, though it remains lawful for a JADU. The city's own page still describes the older position. We confirm the operative rule with Planning rather than designing around a clause that may not be enforceable.
Belmont requirements beyond the ADU code
On a hillside lot the ADU section is rarely the binding constraint. The grading, geologic and tree rules usually are, and none of them carry an ADU exemption.
Grading permits trigger early
A permit is required if combined cut and fill exceeds 50 cubic yards, if any cut or fill is deeper than 2 feet at its deepest point, or if grading and vegetation removal disturbs more than 2,000 sq ft. There is no small-project or ADU exemption.
Above 500 cubic yards it stops being ministerial
Exceed 500 cubic yards, or disturb more than 6,000 sq ft, and the grading goes to the Planning Commission. That converts an otherwise by-right ADU into a public hearing item, with the schedule and cost that implies. Designing to stay under the threshold is usually worth more than it costs.
A recorded covenant before permit
Belmont requires a covenant recorded against the property before the building permit issues, in a form approved by the City Attorney, putting future owners on notice of the sale, occupancy and rental restrictions.
No short-term rentals
Rentals under 30 days are prohibited for the ADU, the JADU and the primary home. Belmont applies it to the whole property, not just the new unit.
Retaining walls cost more than you think
The usual exemption for walls under 4 feet falls away where the wall supports a surcharge, which on a hillside lot carrying a foundation, a driveway or sloped ground above is most of them. Any wall over 3 feet visible from the street also has to be earthtone, articulated and rough-textured. Wire-cut block is ruled out.
Protected trees at 10 inches
Tree removal permits apply from 10 inches diameter at breast height, and grading submittals must show every tree on site plus any tree within 25 feet of the grading on neighboring property. A certified arborist report is commonly required.
What Belmont charges
From Belmont's adopted Master Revenue Schedule. The impact fee picture here is genuinely favorable compared with neighboring cities, but the hillside review fees are where a Belmont budget actually differs.
- Park improvement impact fee, per unit
- $14,975
- Transportation impact fee, per unit
- $8,093
- Geologic review, geotechnical report
- $4,962 + $3,120
- Grading permit, 0 to 50 cubic yards
- $1,170
- Grading permit, over 50 cubic yards
- $1,170 + $3/cu yd
- Tree removal permit, 10 inch and above
- $92
- An ADU under 750 sq ft pays no impact fees. Above that they are charged in proportion to the ADU's size against the primary home. Belmont publishes the worked example itself: an 800 sq ft ADU behind a 2,400 sq ft house pays a third of the single-family rate, which lands around $7,700.
- That is roughly a third of what the same unit would pay in Millbrae. Impact fees vary enormously between neighboring Peninsula cities and it is worth knowing which side of the line you are on.
- The geologic review fees are separate from, and additional to, the building permit. On a mapped hazard parcel they are effectively unavoidable because an ADU cannot use the accessory-structure exception.
- Grading over 50 cubic yards also carries a refundable deposit of $60 per cubic yard, rising to between $50 and $90 per cubic yard where the city engineer identifies a complex geotechnical condition such as landslide, creek improvement, bay mud or the San Juan steep hillside area.
- Building permit fees are valuation based with add-ons that stack to roughly 163% of the base building fee plus 0.75% of valuation. Model them rather than assuming the base figure is the cost.
Belmont's schedule, and the five months you cannot grade
Belmont publishes its plan check times, which is helpful. The constraint that actually moves a hillside ADU's completion date is not plan check at all.
- 14 days
Initial processing
Allow up to fourteen calendar days for the city to process a submittal before plan check time starts running.
- 21 to 28 days
Plan check
Belmont's published current plan check time, with resubmittals and revisions at five to ten days. Plan checkers have discretion to add up to seven calendar days to the posted date.
- 30 days
Pre-approved plans
Belmont runs a pre-approved ADU program. A site-specific application using a previously approved plan is reviewed within thirty days, provided the plan is identical. Pre-approval lapses at the end of each building code cycle.
- Nov 15 to Apr 15
The grading moratorium
This is the one that moves completion dates. Erosion control must be in place by November 15, and grading may neither be started nor continued during the winter rain period through April 15. The exceptions are narrow. Miss the window and hillside foundation work waits five months.
Overlay
Fire: the label came off, the building standards did not
Belmont's fire hazard map was redrawn in 2025 and the result surprised a lot of owners. What changed on paper and what changed in practice are two different things.
The Very High zone is gone
Belmont previously carried 775 acres of Very High fire hazard severity zone. The 2025 state maps removed it entirely. The city now has zero acres Very High, zero High, and about 971 acres rated Moderate, roughly a third of the city, concentrated in the western hills.
But the construction standards were extended down to Moderate
San Mateo Consolidated Fire adopted an ordinance effective January 2026 that applies the California Wildland-Urban Interface Code to buildings in Moderate zones. The Fire Marshal said as much publicly: adding Moderate was how the district preserved stricter construction requirements and defensible space enforcement. So a Belmont hillside ADU still builds to wildland standards.
Tree rules apply regardless of zone
New trees must have a mature drip line at least 10 feet from any combustible structure, with 10 feet of crown to crown separation. For conifers, palms, pepper and eucalyptus that distance rises to 30 feet. These apply anywhere in the district, not only in mapped zones.
Landslide and liquefaction zones are mapped here
State seismic hazard mapping places roughly 1,006 acres of Belmont in an earthquake-induced landslide zone, about a third of the city, and roughly 602 acres in a liquefaction zone. There is no Alquist-Priolo fault zone inside the city limits; the nearest is about a mile west.
None of this rules out building. It does mean a Belmont hillside ADU carries engineering that a flatland unit does not, and that the honest version of a budget says so at the start rather than at the foundation inspection.
Where else we build
Testimonials & Reviews
Real reviews from Bay Area homeowners we’ve built for.
“I am so happy to find ADU San Mateo. The crew are nice, professional, doing a great job while pay attention to details, all with a good price. Highly recommend!”
“Finished work in a timely fashion. Quick to respond.”
“Excellent service highly recommended”
“ADU San Mateo went above and beyond our expectations. They were with us every step of the project, making the experience easy and headache free. ADU San Mateo has great attention to detail and abundance of carpentry knowledge. They always answered our calls and ensured the job site was clean and safe before ending the workday. The team was very professional, clean, and respectful. We contracted ADU San Mateo to build our ADU unit and cannot be happier with the outcome. Would definitely recommend to anyone looking for cost-effective get it right the first time company. Definitely would use for our future projects.”
“Dependable, responsive, quality workmanship and dedication to getting the project completed timely and on or under budget. They treated our home as if it was their own. The outcome exceeded my expectations. Would recommend to my friends and family.”
“I would highly recommend ADU San Mateo to all my friends and family. The crew is efficient, polite and hardworking. They are easy to work with and very competitive with pricing. We would definitely recommend and hire again for future projects.”
“Such exceptional work done, I am beyond pleased with the results! Paid close attention to detail and I was always kept informed. High quality work and excellent service gave us the results we expected. I was completely impressed with their professionalism and customer service. I highly recommend this business!”
“I HIGHLY recommend Eli and his team for any home remodeling projects. They did my staircase and completely did what I have envisioned. I am a perfectionist, so I am glad I met them. They are very detailed oriented, very punctual, and there were no delays. He advised me what’s the best route, up to code, and provided recommendations tweaks on my design. I provided a 3D rough draft sketch to them, the measurements were 100% accurate. I can guarantee you that you won’t be disappointed, as I went through roughly 10 contractors and FINALLY found the right team. I am looking forward to seeing how my kitchen tiles turn out.”
“They were responsive and efficient and easy to work with.”
Belmont ADU questions
Here are some common questions along with their answers to help clear up any confusion.
Still have a questions?
Let’s discuss your goals and find the best ADU solution for your property.
Get a free ConsultationHow big an ADU can I actually build on a Belmont hillside lot?
How tall can a detached ADU be?
Will I need a geotechnical report?
Why does my builder say we have to start before November?
Do I lose parking or need to add any?
Do I have to live on the property?
Where these figures come from
Every rule and fee on this page is taken from Belmont’s own published code and fee schedule. Ordinances change, so confirm anything you are budgeting against.
Your dream ADU starts here
Free feasibility review, no-obligation consultation, response within 1 business day.