Can the city make me open walls to legalize an unpermitted ADU?

It can ask for limited openings, yes. To legalize an unpermitted unit, the building department has to verify that hidden wiring, plumbing and framing meet minimum health and safety standards, and some cities say outright that small areas of drywall may need to come out. State law limits when a city can deny legalization of an ADU built before 2020, but building code and safety corrections remain under local discretion.

What state law says

• Pre-2020 units. Under Government Code section 66311.7, as summarized in HCD's ADU Handbook, a local agency cannot deny a permit for an unpermitted ADU built before January 1, 2020 because it violates building standards or ADU rules. The exceptions: the agency finds that correcting the violation is necessary to protect health and safety, or the building is substandard under Health and Safety Code section 17920.3.
• Published checklist. Agencies must publish a checklist of the substandard conditions in section 17920.3, and tell homeowners they may get a confidential third-party code inspection from a licensed contractor before applying.
• Local discretion. HCD notes that corrections related to the building code and health and safety are under the local agency's discretion, and that applicants who disagree should use the local appeals process.

What cities say about openings

• Brisbane says that during the preliminary inspection, the building inspector will determine what may need to be removed to inspect and verify the construction, mechanical, electrical and plumbing installations.
• Hanford lists possible corrective actions that include a drafted set of plans showing the corrections, framing details, and removing small areas of sheetrock to review electrical or plumbing work, decided case by case.
• Gilroy notes that it inspects the unit regardless of any third-party report.

How to prepare

1. Consider the confidential third-party inspection first. It can show what is likely to be found before you apply.
2. Gather proof of the unit's age, which cities ask for, plus anything showing what is installed: receipts, construction photos and equipment model numbers.
3. Make the panel, attic, crawl space, water heater and plumbing access points easy to reach.
4. Allow time and budget for patching and repainting after targeted openings.

Why openings are sometimes unavoidable

Wiring splices, drain venting and pipe materials are often hidden behind finished walls. When an inspector cannot see them and there is no record of a prior inspection, a small opening may be the only way to confirm what is there.

What this means for your MEP plans

Legalization plans document existing conditions and the corrective work, so the more you can see and photograph, the more accurate the MEP sheets can be. MEP Plans USA prepares MEP plans for unpermitted ADUs showing existing systems and the required upgrades. See the ADU amnesty law and MEP plans and common electrical issues in legalization. Rules vary by city, so confirm with your building department. This is not legal advice.

Sources: hcd.ca.gov, brisbaneca.gov, hanfordca.gov, cityofgilroy.org.