Your contractor sends back a number for the backyard cottage, and it is bigger than you hoped. Before you start cutting finishes, it helps to know that a chunk of an accessory dwelling unit's (ADU — a second, self-contained home on your lot) cost is set by rules, not by lumber prices: fees, utility hookups, sprinklers, parking. In Piedmont, several of those rules work in your favor.
Short answer: Our code library does not publish construction prices, so no one can quote you a Piedmont ADU cost from the code. What the rules do fix: no impact fees on an ADU of 750 square feet or less (§ 66311.5), no fire sprinklers unless your house already has them, and no replacement parking when you convert a garage (17.38.060).
Key numbers
| What it affects | The number | Where it comes from |
|---|---|---|
| Impact fees waived | ADU of 750 sq ft or less (junior ADU 500 sq ft or less) | § 66311.5 |
| No lot coverage, floor-area or landscape limit in Piedmont | ADU of 800 sq ft or less | 17.38.060 |
| Piedmont's maximum size, detached ADU | 850 sq ft, or 1,000 sq ft with more than one bedroom | 17.38.060 |
| Piedmont's minimum size | 150 sq ft | 17.38.060 |
| Piedmont's height limit, detached ADU | 18 ft, plus 2 ft for a roof pitch matching the house | 17.38.060 |
| Side and rear setback, new structure | 4 ft (none for a conversion of an existing structure) | 17.38.060 |
| City-owned plan variation allowed | 3 percent or less in any one dimension | 17.38.075 |
| Rent restriction if you use the city's plans | 10 years, affordable to very low income households | 17.38.075 |
What the code does not tell you
The code does not set, estimate or even mention construction costs per square foot, contractor prices, or what Piedmont's permit fees add up to in dollars. Piedmont's own planning fees are set by city council resolution, not written into the ordinance (17.60.040). For a real number, you need bids from builders and the city's current fee schedule. What follows is the part the rules decide.
Fees: the 750 square foot line
An impact fee is a one-time charge a city or district collects to pay for the extra load a new home puts on parks, roads and similar services. State law says a city, special district or water company cannot charge any impact fee on an ADU with 750 square feet of interior livable space or less, or a junior ADU (a small unit carved out of your existing house) of 500 square feet or less (§ 66311.5).
Above 750 square feet, impact fees are allowed but must be charged proportionately to the size of your main house (§ 66311.5). So a design that lands just over the line can pick up fees a slightly smaller one avoids entirely.
Piedmont's application fees are separate. The city charges either a flat fee or its actual cost of processing, and for actual-cost work it takes a deposit up front. If an outside consultant is needed, you pay the billed amount plus 10 percent for the city's administrative cost, and any unused deposit is refunded within 15 working days after the final decision (17.60.040).
Utility hookups
Utility connections are often a surprise line item. State law limits them in two ways:
- An ADU is not treated as a new residential use when a water or sewer provider calculates connection fees or capacity charges, unless the ADU is built at the same time as a new single-family home (§ 66311.5).
- If you convert space inside your house or an existing accessory building (the kind of project described in § 66323), the provider cannot require a new, separate utility line or charge a related connection fee (§ 66311.5). Piedmont's ordinance repeats this (17.38.060).
For a new detached ADU, a separate connection can be required. Any fee must be proportionate to the unit's burden on the system, based on its square footage or its plumbing fixture count, and cannot exceed the reasonable cost of the service (§ 66311.5).
Things Piedmont cannot make you build
Several rules keep the scope of work down:
"Accessory dwelling units shall not be required to have fire sprinklers if they are not required for the primary residence." — Piedmont 17.38.060
Sprinklers count as required only if your house already has them or a new house is being built alongside the ADU. Building the ADU also cannot trigger sprinklers in your existing home (17.38.060).
- Parking: if you tear down or convert a garage or carport for the ADU, the city cannot require you to replace those parking spaces (17.38.060).
- Old problems: the city cannot make you fix nonconforming zoning conditions, code violations or unpermitted structures that are unaffected by the project and do not threaten health and safety (17.38.060).
- Passageway and lot size: no passageway and no minimum lot size can be required (17.38.060).
Size is the biggest lever
Square footage drives cost more than anything else, and the rules cluster around a few sizes. In Piedmont, a detached ADU can be up to 850 square feet, or 1,000 square feet with more than one bedroom. An attached ADU is also capped at 50 percent of your existing living area, but never forced below 800 square feet (17.38.060).
At 800 square feet or less, Piedmont applies no maximum lot coverage, no floor area ratio (the cap on total building area compared to lot size) and no minimum landscape area (17.38.060). Combined with the 750 square foot fee line, many owners will find the 750 to 800 square foot range worth a close look with their designer.
These local limits sit on top of a state floor. State law bars a city from capping ADU size below 850 square feet, or 1,000 square feet for more than one bedroom, and from using lot coverage or similar limits to block an 800 square foot ADU with four-foot side and rear setbacks (§ 66321). That is a limit on the city, not a size you must build.
City-owned plans, with a condition
Piedmont owns a set of council-approved ADU floor plans and elevations. You can use them to get planning approval if your unit meets the development standards, uses the same roofing and siding as your house, and stays within 3 percent of the plans in any dimension (17.38.075). That can shorten the design phase.
The trade-off is real. If approved this way, you must record a rent restriction for 10 years, keeping rent affordable to very low income households, and certify the rent to the city every year by December 31 (17.38.075). "Very low income" means a household earning between 30 and 50 percent of the Alameda County median (17.38.020). If you plan to house family or charge market rent, factor that in.
Where to go next
Get at least two builder bids, ask Piedmont for its current fee resolution, and ask your water and sewer providers what they will charge before you finalize the size. The Piedmont overview collects the city's other rules in one place.
Sources
- California Government Code § 66311.5 — fees for ADUs
- California Government Code § 66321 — limits on local size and height rules
- California Government Code § 66323 — ADUs cities must approve ministerially
- Piedmont 17.38.060 — ADU development standards
- Piedmont 17.38.075 — pre-approved plans incentive
- Piedmont 17.38.020 — ADU definitions
- Piedmont 17.60.040 — fees and deposits
