Mountain View ADU and Zoning Guide for Homeowners

Last updated November 13, 2026. Reviewed by the Cash Home Buyers CA team.

Mountain View has a process to legalize existing unpermitted ADUs, and the city says no code enforcement action or penalties will be assessed on those units. It also runs a preapproved ADU program for new detached units.

Legalizing an existing Mountain View ADU

Eligibility	Built or converted before January 1, 2020, no existing building permit for the unit, and zoning that allows an ADU
What is required	Submit a building permit application and correct existing code violations that do not meet current building or zoning codes
Penalties	The city says no penalty or violation fees will be charged
Plans	Must comply with the city’s ADU standards
Building contact	(650) 903-6313, building@mountainview.gov
Planning contact	(650) 903-6306, planning.division@mountainview.gov
Location	City Hall, First Floor, 500 Castro Street

The legalization steps

1. Review zoning and building code requirements.
2. Contact Planning and Building with questions.
3. Draft plans with an architect or engineer.
4. Submit a building permit application; more than one review round is possible.
5. Build, schedule inspections online, and finish all required inspections to make the unit legal.

Preapproved ADUs

A preapproved ADU is a detached, new-construction unit from a third-party designer that the city has already reviewed for building, fire and reach code compliance.

Who qualifies to legalize an existing Mountain View ADU

Mountain View’s legalization page sets three conditions: the unit was built or converted before January 1, 2020, there is no existing building permit for it, and the property’s zoning allows an ADU. The city says owners who legalize an ADU or JADU through this process are not charged penalty or violation fees. Permit fee estimates are posted on the city’s ADU page.

The legalization steps

1. Review the regulations. Read the zoning and building code requirements to decide whether the program fits.
2. Contact the city. Planning handles ADU zoning questions and the Building Division handles code and permit requirements.
3. Research the costs and benefits. The city suggests consulting a tax advisor about property tax implications.
4. Draft plans. An architect or engineer prepares building plans that comply with the city’s ADU standards.
5. Submit. File a building permit application with the Building Inspection Division.
6. Permit issuance. Plans may need more than one round of review before a permit is issued.
7. Construct and inspect. After the permit is issued, work begins and inspections can be scheduled online.
8. Unit becomes legal. Once construction and all required inspections are complete, the unit is legal.

City Hall	500 Castro Street, first floor, Mountain View, CA 94041
Building	(650) 903-6313, building@mountainview.gov
Planning	(650) 903-6306, planning.division@mountainview.gov
Public Works	(650) 903-6311, public.works@mountainview.gov

Pre-approved ADU plans

For owners who want to build a new ADU, Mountain View offers a preapproved ADU program, with plans that can be viewed and selected on the ADU Accelerator website. The city lists these figures:

• Plan check fee: $1,250
• Typical permit fees including inspections: $7,000 to $13,000, based on a construction valuation of $100,000 to $300,000
• Excavation permit fee: up to $12,000
• Preapproved ADU permit review: up to 30 days, compared with 60 to 90 days for a standard ADU permit
• City staff review of an initial application or a resubmittal: 7 business days

A permit submittal includes a copy of the preapproved plans, the Preapproved ADU Information Form, a site plan, a Title 24 energy report if it is not included or has been altered, a site survey if structures are within 6 feet of a property line or required setback, and a change of address request form.

What this means for a sale

An unpermitted unit can complicate a conventional sale because lenders and appraisers may not count it, and because you may need to disclose it.

California Civil Code section 1102.1 keeps your duty to disclose material facts in an as-is sale, and under section 1102.3 a buyer who receives a late disclosure can cancel within three days of in-person delivery or five days of mailing. Our as-is selling guide covers the steps.

If the unit was built or converted before 2020, legalizing it first may widen your buyer pool, but it needs plans, review rounds and inspections. A direct cash buyer can purchase the property as it stands.

Points to weigh before you decide

• Time: plans may need several review rounds, and a permit is only the start of construction and inspections.
• Cost: plan preparation, permit fees and corrective work are yours.
• Tax: the city advises talking to a tax advisor about property tax effects. Our Prop 13 reassessment guide explains what triggers a reassessment.

Property tax and hazard disclosure for a Mountain View home with an ADU

The city advises talking to a tax advisor about property tax implications, and the state rules are worth knowing: Proposition 13 taxes a home at 1% of taxable value plus voter-approved debt, with yearly increases in base year value generally limited to 2% until a change in ownership or new construction. Ask the Santa Clara County Assessor how legalizing a unit would be treated.

California also requires sellers to disclose six natural hazard zone types, including flood, dam inundation, very high fire hazard, earthquake fault and seismic hazard zones and state responsibility wildland areas. The seller or the seller’s agent must deliver the disclosure. See our hazard disclosure guide.

Closing costs to weigh against legalization

The county documentary transfer tax is $0.55 per $500 of value or fraction, with a possible city tax of half that amount. On $600,000 the county amount is $660. Loan payoff, escrow, title, any commission and the proration of property tax are also deducted. Weigh these against the plan, permit and construction costs for legalization, using the calculator in our seller closing costs guide.

How to sell a house in Mountain View

1. Find out whether any unit or addition is unpermitted.
2. Decide whether to legalize first or sell as-is.
3. Request a no-obligation cash offer.
4. Pick your closing date.
5. Close with no repairs required.

Frequently asked questions

Will Mountain View penalize me for an unpermitted ADU?

The city says no code enforcement action or penalties will be assessed on unpermitted ADUs that go through its legalization process, and no penalty or violation fees are charged.

Which ADUs qualify for legalization?

Units built or converted before January 1, 2020, with no existing building permit, on property whose zoning allows an ADU.

Will Mountain View charge penalties to legalize an ADU?

The city states that property owners who legalize an ADU or JADU through its process are not charged penalty or violation fees. Normal permit fees still apply.

How long does a Mountain View preapproved ADU permit take?

The city lists up to 30 days for a preapproved ADU permit review, compared with 60 to 90 days for a standard ADU permit.

What does a Mountain View preapproved ADU plan check cost?

The city lists a $1,250 plan check fee, with typical permit fees including inspections of $7,000 to $13,000 for construction valuations of $100,000 to $300,000.

More guides

• We Buy Houses in Santa Clara County
• Santa Clara County Property Tax Guide
• All California Guides

Sources

• City of Mountain View: Legalize Existing ADU
• City of Mountain View: Preapproved ADU Program

This guide is general information from public sources, not legal advice. Confirm current rules with the City of Mountain View.