800 sq ft ADU · checked Oct 18, 2026
800 sq ft ADU fees in Mountain View: $8,646–$49,845, $2,778 more than at 750 sq ft
At 800 sq ft an ADU crosses the 750 sq ft line, so Mountain View’s impact fees apply in proportion to the main home: $2,509 with a 2,000 sq ft house, against $0 at 750 sq ft. From January 1, 2027, SB 1117 limits them to the 50 sq ft over 750, which our model puts at $157.
750 vs 800
What the extra 50 sq ft changes in Mountain View
- Citywide transportation fee: $0 at 750 sq ft → $1,406–$2,509 at 800 sq ft (exempt at 750)
- School impact fees (Mountain View Whisman / Los Altos & MVLA high school districts): $0–$4,035 at 750 sq ft → $0–$4,304 at 800 sq ft
Staying at 750 sq ft would save about $2,778 in fees under today’s law.
Line by line: 750 sq ft vs 800 sq ft, now and under SB 1117
Detached ADU, 2,000 sq ft main home (example assumption). The 2027 column applies SB 1117, which limits impact fees to the area over 750 sq ft.
| Fee | 750 sq ft (2026) | 800 sq ft (2026) | 800 sq ft (from 2027) |
|---|---|---|---|
| Planning & zoning | |||
| Land use document fee (0.24% of construction valuation) | Est.$240–$720 | Est.$240–$720 | Est.$240–$720 |
| Building permit & plan check | |||
| Building permit, new construction ADU (City's typical range, incl. inspections) | Est.$7,000–$13,000 | Est.$7,000–$13,000 | Est.$7,000–$13,000 |
| Excavation permit, if utility or right-of-way work is needed | May apply$12,000 | May apply$12,000 | May apply$12,000 |
| Impact fees | |||
| Citywide transportation fee | Exempt | Est.$1,406–$2,509 | Est.$88–$157 |
| School district fee | |||
| School impact fees (Mountain View Whisman / Los Altos & MVLA high school districts) | Up to$4,035 | Up to$4,304 | Up to$4,304 |
| Utility connections | |||
| Water capacity fee (if a new connection is required) | May apply$9,389 | May apply$9,389 | May apply$9,389 |
| Sewer capacity fee (if a new connection is required) | May apply$7,923 | May apply$7,923 | May apply$7,923 |
| Modeled total | $7,240–$47,067 | $8,646–$49,845 | $7,328–$47,493 |
State law
Why 800 sq ft is the size every city must allow
Local limits on lot coverage, floor area ratio, open space and minimum lot size can’t stop an 800 sq ft ADU with 4 ft side and rear setbacks, and a detached unit of up to 800 sq ft must be approved ministerially alongside a junior ADU.
Mountain View: Single-family lots: 850 sq ft (studio or 1 bedroom) or 1,000 sq ft (2+ bedrooms); an attached ADU is capped at 50% of the main home. Multifamily lots: 1,200 sq ft detached.
Gov. Code §66321(b)(3)
size based upon a percentage of the proposed or existing primary dwelling, or limits on lot coverage, floor area ratio, open space, front setbacks, and minimum lot size, for either attached or detached dwellings that does not permit an accessory dwelling unit with at least 800 square feet of interior livable space and with four-foot side and rear yard setbacks to be constructed in compliance with all other local development standards.
Gov. Code §66323(a)(2)
One detached, new construction, accessory dwelling unit that does not exceed four-foot side and rear yard setbacks for a lot with a proposed or existing single-family dwelling.
Gov. Code §66323(a)(2)(A)
A total floor area limitation of not more than 800 square feet of livable space.
Quoted from Government Code, Title 7, Division 1, Chapter 13 (Accessory Dwelling Units), incl. §66311.5 (opens in new tab), checked Oct 18, 2026.
FAQ
Questions about 800 sq ft adu in Mountain View
How much are the fees for an 800 sq ft ADU in Mountain View?
Our model of Mountain View’s official schedules comes to $8,646–$49,845 for a detached 800 sq ft ADU with a 2,000 sq ft main home, compared with $7,240–$47,067 at 750 sq ft.
Does an 800 sq ft ADU pay impact fees in Mountain View?
Yes: $2,509 in our model, because 800 sq ft is over the 750 sq ft exemption. Fees must be proportional to the main home; a larger house means a smaller share. From 2027, SB 1117 limits them to the 50 sq ft over 750 ($157 in our model).
Can Mountain View stop me from building an 800 sq ft ADU?
Not with lot coverage, floor area ratio, open space or minimum lot size limits. State law requires cities to allow at least an 800 sq ft ADU with 4 ft side and rear setbacks (Gov. Code §66321(b)(3)), provided it meets other objective standards such as height.
Do school fees apply at 800 sq ft?
Yes. School fees apply above 500 sq ft; at 800 sq ft the statewide Level 1 cap is $4,304, and many districts charge less.
Not modeled yet
What these totals leave out
- Building permit: the City publishes only a typical range ($7,000–$13,000 for new construction), not a formula.
- Which transportation-fee and capacity-fee class applies to an ADU.
- Park land dedication in-lieu fee (land-value based; ADU treatment not stated).
- School district rate.
More
Keep going
Sources
Official sources
Every figure comes from these documents; each line’s quoted text is on the Mountain View page.
- City of Mountain View, Development PermitsAccessory Dwelling Units (permit guide) (opens in new tab)
- City of Mountain ViewDevelopment Fees (effective August 22, 2026) (opens in new tab)
- California State Allocation Board (Department of General Services, OPSC)State Allocation Board item: 2026 adjustment to the Level I developer fee (meeting of January 28, 2026) (opens in new tab)
- California Legislative Information (leginfo.legislature.ca.gov)Government Code, Title 7, Division 1, Chapter 13 (Accessory Dwelling Units), incl. §66311.5 (opens in new tab)
- HCD via data.ca.govHousing Element Annual Progress Report (APR) data, Table A2: ADU building permits by year (opens in new tab)