News · September 30, 2026 · In PG&E's September 30, 2026 report, finished jobs that needed only a new service line averaged 307 days from application to power, including 113 days on PG&E's own steps. [1]
The rule everyone hears is 4 feet. California lets a new backyard ADU sit as close as 4 feet to the side and rear lot lines, and a city must approve one of up to 800 square feet that meets the state's limits [2]. From January 2027, that's two of them [3]. So owners picture the cottage tucked into the back corner, where it takes the least yard.
Take an example lot in San Mateo, 50 by 150 feet, with the house up front. Just over the back fence stands the neighbor's oak, 10 inches across. Four things decide whether that corner works, and it pays to check them before anyone draws.
Can a protected tree, even the neighbor's, limit where an ADU goes?
It can. In San Mateo, an oak is a protected heritage tree from 10 inches across, measured 54 inches above the ground [4]. Cutting its roots within six times the trunk's diameter needs a permit. Construction within ten times needs a tree protection plan by a certified arborist before the building permit is issued [4]. For a 10-inch oak, those rings are 5 feet and 8 feet 4 inches from the trunk. The city's guidelines say the plan covers trees on neighboring property too [5].
On the example lot, the oak stands 2 feet past the fence. A cottage at the 4-foot line puts its back wall 6 feet from the trunk: outside the root-cutting ring, inside the protection zone. That means an arborist's plan, and digging footings inside the zone only if the city arborist approves [4].
Does a city's tree rule even apply to the ADU the state makes it approve? State ADU law never mentions trees. For the ADUs a city must approve, it bars local standards the statute doesn't list [2]; for other ADUs, a city's ADU ordinance may set objective "landscape" standards [6]. In 2023, HCD told Santa Clarita to remove a rule applying its oak ordinance to ADUs [7]. Palo Alto changed its code in 2024 so a protected tree may be removed to make room for an ADU under its state-standard rules [8]. San Carlos's code still applies its tree rules to second units [9], and San Mateo's code doesn't say. Los Altos's checklists for its pre-approved and permit-ready ADU plans ask for an arborist report when work reaches the inner two-thirds of a protected tree's dripline [10][11]. We know of no court ruling on it, so ask your city in writing. And the neighbor's tree is the neighbor's, even where its roots reach into your yard [12].
The cost to the household: an arborist's report before design, and a cottage that may move a few feet toward the middle of the yard.
How much must an ADU's sewer slope, and when does it need a pump?
The plumbing code wants a building sewer to slope at least a quarter inch per foot toward the sewer it drains to [13]. From the back corner of a 150-foot lot, the run can be 120 feet, which takes 30 inches of fall. Only with the code official's approval, and where the street sewer's depth or the building's structure or layout makes that slope impractical, may a 4- to 6-inch line slope an eighth of an inch per foot [13].
A yard that slopes down toward the back eats that margin fast. Fixtures below the top of the main sewer must drain to a sump and be pumped up [13]. Fixtures on a floor lower than the next upstream manhole cover need a backwater valve [13], and some districts, such as West Bay, which serves most of Menlo Park and parts of Atherton, set that line a foot higher [14][15][16]. For new buildings, Cupertino Sanitary District requires a pump when the lowest floor with fixtures sits less than a foot above that manhole's rim [17].
Then there's the tie-in. State law lets a city or district require a separate sewer connection for a new detached ADU, with a fee proportional to its size or its plumbing fixtures, and the 750-square-foot fee exemption doesn't cover connection charges [18]. Whether the cottage may share the house's line is up to the local sewer agency, the city or a sanitary district. In Hillsborough, building a new structure that needs a sewer connection on a lot that already has a private lateral (the pipe from the house to the public sewer) puts that lateral up for evaluation, and repair or replacement if needed [19]. In San Carlos, work that adds toilets needs the lateral to pass inspection before occupancy, unless a current certificate is on file [20].
If the slope isn't there, the cottage gets a pump and valves, and that's cheapest to learn before the floor height is set.
Who brings power to a new ADU, and how long does it take?
PG&E delivers power in 11 of the 12 cities, and Palo Alto runs its own utility [21][22]. When PG&E builds a new service, the owner is responsible for a clear route, the trenching and the conduit on the property [23]. PG&E's allowance is $3,255 per meter or dwelling unit, and since July 2024 there is none for new buildings that also use gas or propane, apart from some projects that were in design review by then and are energized by June 30, 2027 [24]. Sewer and water lines can't share PG&E's trench [25].
The bigger cost is time. PG&E's own posting puts its average invoice for an ADU's new electric service at $4,831.86, with 303 days from application to power, for jobs finished by August 2025 [26]. The state's targets count only PG&E's steps: 182 days on average for a service line, and no more than 335 for 95 percent of jobs [27][28]. The newest report, in the box above, shows 307 days start to finish [1].
In Palo Alto, the utility normally allows one service per lot. A cottage gets its own meter by swapping the house's meter panel for a two-meter panel, and it needs its own address [22][29]. The city's standard fee for a single-phase underground service up to 200 amps is $4,093 when no transformer upgrade is needed, before your trench [30].
So ask the utility as soon as the spot is set.
Fire hazard zone maps changed in 2025, and so did the building rules
The State Fire Marshal released new local fire hazard maps for San Mateo and Santa Clara counties on February 24, 2025, with Moderate, High and Very High zones [31][32], and the city or its fire district had 120 days from receiving them to adopt them [33]. Among our 12 cities, Very High land is only in Palo Alto, Cupertino and Saratoga. Hillsborough, San Carlos, Redwood City and Los Altos have High and Moderate. San Mateo and Belmont have Moderate only, and Menlo Park, Atherton and Mountain View have none [32].
From January 1, 2026, an emergency rule extended the state's wildfire building code from Very High zones to local High zones as well [34]. The Commission readopted that rule for 90 days in May 2026, and we found no record of a second readoption [35][36]; the permanent version it took up in August 2026 takes effect July 1, 2027 [37][38]. So ask the building department which rules apply on your permit date. Saratoga applies the code in Moderate zones too, and so does San Mateo Consolidated Fire, which serves San Mateo and Belmont [39][40][41]. A new ADU in a covered zone needs, among other things, a Class A roof, ember-resistant vents (listed or tested types, or fine noncombustible mesh on roof vents), and windows with a tempered pane or another approved glazing. Under the 2025 code, its gable-end and dormer vents must sit at least 10 feet from lot lines [42], which rules them out on the wall facing a line 4 feet away; the permanent revision, from July 2027, applies the 10-foot rule to roof ridge and off-ridge vents instead [37].
A fire zone alone doesn't bar an ADU. HCD says a High zone "is not a conclusive rationale" for restricting ADUs, and a city's area limits can't preclude the ADUs it must approve [43]. In Very High zones, state law also requires 100 feet of defensible space, up to the property line [44]. The Board of Forestry's 5-foot "Zone 0" rule, approved August 19, 2026 [45], was withdrawn from state review on September 8 and isn't in force [46].
What to check first, in order, before you place an ADU
The fire map. It's a lookup, and it sets how the cottage is built.
The trees. A tape measure and the city's rule show how close you can dig.
The sewer. The elevations of the cottage floor and the sewer set the floor height, or a pump.
The power. Ask the utility as soon as the spot is set; its answer comes last.
The city, in writing. Ask which of these rules it applies to your ADU.
Amiari Construction builds ADUs and additions on the Peninsula.
How we checked this: In October 2026, Amiari Construction read the state laws in the legislature's own data, HCD's handbook and letters, the 2025 plumbing code, the wildfire code's rulemaking texts, PG&E's and Palo Alto's utility rules and reports, the State Fire Marshal's 2025 maps, and tree, sewer and fire records for the 12 cities and their districts.
