Homeowners in Danville planning an accessory dwelling unit almost always brace for the wrong fight. They picture a Planning Commission hearing, a neighbor's objection, a zoning administrator citing lot coverage or setback minimums that kill the project before it starts. That fight is already over. California's ADU statutes, principally Government Code Section 65852.2 along with SB 9, AB 68 and AB 881, require Danville to approve a qualifying ADU regardless of local zoning. The Town cannot reject a detached unit, a garage conversion, or a junior ADU on the grounds that the lot is too small, the setbacks don't work, or the property sits in single-family zoning. Many owners assume those three things disqualify them. Usually they don't.
That preemption is real and it matters, but it also creates a blind spot. Because the zoning objection has been taken off the table, homeowners stop expecting resistance anywhere else in the process. The resistance that remains is procedural rather than political, and it shows up in two places most people never think to research before they call a contractor: Danville's Historic Preservation Commission, and the Central Contra Costa Sanitary District.
What the historic district actually blocks
Downtown Danville sits inside the Hensley Historic District, listed with the California Office of Historic Preservation. The Town's local preservation ordinance requires a Certificate of Appropriateness for exterior work on a contributing structure, but it carves out an exception for minor work: alterations that don't change a property's historic significance, use, intensity, general character, architectural style, or circulation don't need a Historic Preservation permit at all.
That exception is the reason a garage conversion inside the historic district tends to move faster than owners expect. A 1920s-era home on a quarter-acre corner lot with a detached single-car garage, converting that garage into a junior ADU with a kitchenette but no stove, no added square footage, and the existing footprint untouched, typically clears design review in one to two weeks with no added fee, because the exterior scope of work doesn't rise to the level the ordinance is built to regulate. The commission exists to stop demolition, facade changes, and additions that alter what makes the district legible as a district. It isn't built to stop a homeowner from adding a bathroom fixture behind a garage door that already faces the street.
What the ADU actually costs, and why the number moves
Danville's ADU rules apply the same way whether the lot sits flat in the historic core or climbs a hillside in Blackhawk or the Danville Hill area, but the geotechnical reality of the two settings produces very different budgets. A junior ADU built inside an existing garage envelope, requiring no new foundation and reduced electrical work, ran roughly this range as of this spring:
| Item | Typical range |
|---|---|
| Plans and design | $1,500 to $2,500 |
| Permit fee | $2,200 to $2,800 |
| Utility sub-metering | $800 to $1,200 |
| Construction | $30,000 to $45,000 |
| Total project cost | $38,500 to $48,500 |
| Timeline, plans through completion | 13 to 18 weeks |
A hillside ADU built above an existing garage in a neighborhood with 40 feet or more of elevation gain looks nothing like that. Expansive clay soil common to those slopes carries a real probability of settlement, frost depth on those lots typically runs 18 to 24 inches deeper than flat-lot construction requires, and a geotechnical report becomes a practical necessity rather than a formality. None of that is a zoning objection. It's an engineering cost that state ADU law never touched, because state law addresses whether Danville can say no, not what the ground underneath the project actually requires.
The sewer permit nobody budgets for
Danville sits entirely within Central San's service territory, the same district that also serves Alamo, Lafayette, Moraga, Orinda, Pleasant Hill, Walnut Creek, and parts of Martinez and San Ramon. Central San issues sewer permits separately from the Town's building permit, and for ADU projects specifically, the rules change depending on how the unit is built:
- If the ADU is attached to or contained within the existing house, no separate sewer line is required at all.
- If the ADU is a separate, detached structure, its sewer line cannot tie into the house's internal plumbing. It has to connect to the house's existing lateral somewhere on the property, downstream of the main cleanout.
- If the ADU is part of an SB 9 lot split, it needs its own independent connection into the public sewer main, not a shared lateral with the original house.
That's three different plumbing problems hiding inside what looks like a single planning decision, and Central San's own plan review currently runs a minimum of 10 to 25 business days on top of whatever the Town's building review takes. Danville's own permit checklist reflects this sequencing: for additions that expand a building's footprint, or for any new attached or detached structure including an ADU, the applicant needs Central San's stamp on the plans before the Town will even accept the building permit application. Homeowners who plan their ADU timeline around the Town's building department alone are missing a dependency that sits earlier in the sequence, not later.
The inspection that never happens, until it does
Central San states plainly that it does not have a point of sale sewer lateral inspection ordinance. An inspection is encouraged before a sale, not required, and no compliance certificate has to be filed before escrow closes on a Danville property, in contrast to districts like EBMUD to the west, where a certificate, and in some cases a refundable deposit, is part of the closing process for cities like Oakland, Alameda, Albany, Emeryville and Piedmont.
That absence of a formal check is usually framed as good news for Danville sellers, and in the sense that there's no fee or city portal to navigate, it is. But the property owner remains fully responsible for the entire lateral, from the house to the point it connects to Central San's main line, including the segment running under the sidewalk and street. Add a new ADU connection to an older lateral and there's no district inspector who checks whether that additional load is something the existing pipe can actually handle before the property changes hands. If a buyer's own inspector finds a problem during their contingency period, that becomes a private negotiation over credits or repairs with no set process and no defined timeline, rather than a compliance step both sides already expected.
A few questions we hear often
Does building in the Hensley Historic District mean I need Planning Commission approval for an ADU? Not automatically. Minor alterations that don't change the exterior character of a contributing structure can be exempt from Historic Preservation review entirely. A garage conversion that keeps the existing footprint and street-facing appearance is the kind of project this exemption is written for.
Is a detached ADU harder to permit than one built inside my existing garage? The zoning approval is the same either way, since state law removes that variable. The sewer connection is not the same. A detached unit needs its own tie-in to the existing lateral rather than sharing the house's internal plumbing, which adds a separate Central San permit and review window that an attached conversion doesn't require.
If I'm planning to sell soon, should I get my sewer lateral inspected before I add an ADU? Central San doesn't require it, but that's different from it being irrelevant. Adding load to an aging lateral without knowing its condition means any problem surfaces during a buyer's inspection period instead of before you've committed to the project.
Does an SB 9 lot split change the sewer requirements? Yes. An ADU that's part of an SB 9 lot split needs its own independent connection to the public sewer main rather than sharing the original house's lateral, which is a different scope and cost than a standard ADU on an undivided lot.
If you're weighing an ADU addition before you list a Danville property, or looking at a home that already has one, the sequence matters as much as the square footage. The Dunn Team can walk through what an existing or planned ADU means for your specific lot, your timeline, and your sale, before you're mid-permit and finding out the hard way.
