Mountain View / ADU construction
ADU Construction in Mountain View
Mountain View allows a 28 foot, two-story ADU. Most of the wait is the permit.
Mountain View permitted 86 ADUs in 2025 and allows two-story units up to 28 feet, more than many neighbors. We design and build ADUs here with the city's 4 to 6 month permit path, the site survey and the utility work planned before the first drawing.
THREE WAYS TO ADD A UNIT
Custom ADU, preapproved ADU or junior ADU.
Mountain View's rules, fees and review times differ by type. The right one depends on the lot, the budget and how fast you need the permit.
| Custom ADU | Preapproved ADU | Junior ADU | |
|---|---|---|---|
| Size | 850 or 1,000 sq ft by bedrooms | Fixed design | Up to 500 sq ft |
| Where | Attached or detached | Detached only | Inside the home |
| City review | 60 to 90 days | About 30 days | Standard review |
| Design changes | Fully custom | Floor plan fixed | Within existing walls |
| Owner on site | Not required | Not required | Required, deed restriction |
| Planning permit | Not required | Not required | Not required |
Source: City of Mountain View ADU and preapproved ADU pages ↗
MOUNTAIN VIEW ADU STANDARDS
Size follows the bedroom count.
On a single-family lot, a studio or one-bedroom ADU can reach 850 square feet and a unit with two or more bedrooms 1,000 square feet. An attached ADU is also capped at 50 percent of the main home. Side and rear setbacks are 4 feet, and one or two stories may reach 28 feet.
Detached ADU
850 square feet for a studio or one-bedroom, 1,000 for two bedrooms or more, 4 foot side and rear setbacks, up to 28 feet for one or two stories.
Attached ADU
Same size caps and no more than 50 percent of the main home's floor area. Internal access is allowed in addition to a separate entrance.
Garage conversion
Converting existing space follows the conversion fee schedule. School impact fees apply once new or converted area exceeds 500 square feet.
Junior ADU
Up to 500 square feet inside the home, with an efficiency kitchen. An owner must live on site, recorded by deed restriction before the permit.
Preapproved plans
City-reviewed designs get permits in about 30 days instead of 60 to 90, because only the site, zoning and utilities are reviewed.
Multifamily lots
Up to 8 detached ADUs on a lot with existing apartments, not more than the existing units, at up to 1,200 square feet each.
MOUNTAIN VIEW AT A GLANCE
Who does what for a Mountain View ADU.
Planning checks zoning, the Building Division issues one combined permit, Public Works handles anything in the street, and PG&E sets the service. Each one can add weeks, so they run in parallel.
- 86
- ADUs issued building permits in 2025
- 4 to 6
- months typical time to receive an ADU permit
- 30
- days typical review for a preapproved ADU
Zoning
Planning Division
No Planning permit is required for an ADU, but the city encourages a preliminary design review with Planning before detailed plans, to catch zoning issues early.
(650) 903-6306Mountain View ADU standards ↗Permits
Building Division and ePermitsMV
One building permit covers electrical, mechanical and plumbing, plus a demolition permit. Each review cycle takes about 2 weeks. Preapproved designs are listed on the city's ADU Accelerator site.
(650) 903-6313Preapproved ADU program ↗Public Works
Excavation and sewer
New or upgraded utility connections and work in the sidewalk or street need an excavation permit, obtained with the building permit. The city also requires a sewer lateral inspection.
(650) 903-6311Trees
Heritage tree rules
Trees with a 48 inch trunk circumference, or 12 inches for oak, redwood and cedar, measured 54 inches above grade, are heritage trees and need a removal permit.
Mountain View trees ↗
BEFORE YOU APPLY
What Mountain View asks for.
The city's ADU submittal is a full plan set in one PDF, in a required sheet order. Missing items restart the 2 week review clock.
Documents to gather
- Title report or recent grant deed
- Site survey if the ADU is within 6 feet of a property line or setback
- Mountain View Green Building residential worksheet
- Structural, energy and electrical load calculations
- Change of Address Request Form
During construction
- Survey certificate approved before the foundation pour
- Demolition permit before any demolition
- Construction and demolition waste tracking account
- Fire sprinkler permit if sprinklers are required
Expect a longer path when
- A heritage tree stands near the building site
- New utilities cross the sidewalk or street
- The existing unit is unpermitted and needs legalization
- A balcony or second story faces a neighbor
Planning the work
How a Mountain View ADU gets built.
Site check
Free visit. Lot, setbacks, heritage trees, sewer lateral, utilities and whether a preapproved plan fits.
Design and Planning check
Plans drawn or a preapproved design adapted. The city encourages a preliminary review with Planning before detailed drawings.
Permit
ePermitsMV submittal with structural, energy and load calculations. Each review cycle takes about 2 weeks; typical total is 4 to 6 months.
Build and final
Survey certificate before the foundation pour, rough-in inspections, finishes and the final inspection that grants occupancy.
WHAT SLOWS ADUS IN MOUNTAIN VIEW
Problems Mountain View owners run into, and how we plan around them.
Collected from Mountain View residents' posts, local news and the city's ADU pages.
The city is described as slow on construction approvals, and permits run past published estimates.
A preliminary Planning review comes first, and the full set is submitted complete in the city's sheet order, so review cycles are not wasted.
The main house is already at its floor area limit.
We check how the city counts the ADU toward floor area on your lot before design, and size the unit to the 850 or 1,000 square foot caps.
A heritage tree removal permit is denied, even with an arborist's report.
Trees of 48 inch circumference, or 12 inches for oak, redwood and cedar, are mapped at the site visit and the ADU is placed around them.
PG&E service work means trenching through the city sidewalk, with special permits and insurance.
The utility route is settled during design, and the excavation permit is obtained in tandem with the building permit, as the city requires.
A garage conversion buyer is unsure which agency even issues the permit.
Mountain View issues its own building permits through ePermitsMV, not the county; we handle the application and every city comment.
Sources: r/mountainview threads, 2024 to 2026; developmentpermits.mountainview.gov ADU, tree and preapproved ADU pages.
How big can an ADU be in Mountain View?
850 square feet for a studio or one-bedroom and 1,000 square feet for two or more bedrooms, on a single-family lot. An attached ADU also cannot exceed 50 percent of the main home. A junior ADU is limited to 500 square feet, and detached ADUs on multifamily lots can reach 1,200 square feet.
Can I build a two-story ADU in Mountain View?
Yes. Mountain View allows attached or detached ADUs of one or two stories up to 28 feet, with a basement counting as a story. An ADU on the second story of an accessory structure is also allowed at 28 feet. Roof decks are prohibited.
How long does an ADU permit take in Mountain View?
4 to 6 months is the city's typical time to receive a permit for a custom ADU, with each review cycle taking about 2 weeks. A preapproved ADU design cuts the review to about 30 days, compared with 60 to 90 days for a standard ADU review.
What are the city permit fees for an ADU in Mountain View?
The city lists building permit fees of $7,000 to $13,000 for a new ADU, $3,000 to $5,000 for a conversion and $4,000 to $6,000 for a junior ADU, plus an excavation permit of up to $12,000 when utility or right-of-way work is needed. School impact fees apply above 500 square feet.
Does an ADU need parking in Mountain View?
Often no. A studio ADU needs no space, and a one-bedroom or larger needs one space unless it qualifies under City Code Section 36.12.95 or replaces an uncovered space demolished to build it. A junior ADU needs no parking.
Can I legalize an unpermitted ADU in Mountain View?
Yes, if it was built before January 1, 2020, has no existing permit and sits on a lot where ADUs are allowed. Mountain View charges no penalty or violation fees for units brought through its legalization process, but code violations must be corrected.
