Building an ADU in Hidden Hills. Rules, costs, timeline.

Can I build an ADU in Hidden Hills?

Everything in this section comes from HHMC Article P (§§ 5-2P-1 to 5-2P-9), enacted by Ordinance No. 407 and effective January 12, 2026, read together with the RA-S zone standards at §§ 5-2C-2 to 5-2C-4 and cross-checked against Gov. Code §§ 66310–66342 and the HCD ADU Handbook (March 2026).

Five things to know before you start

The five points a planner would raise first, before any regulation gets explained.

How many units a Hidden Hills lot can carry

The ordinance splits ADUs into two classes, and the distinction does real work. Class 1 units are the ones Gov. Code § 66323 requires every city to permit; Class 2 units are the ones Hidden Hills regulates locally under §§ 66314–66322 (§ 5-2P-5). Class 1 units are exempt from the local development standards in § 5-2P-7 — the size ceilings, the 50-foot front setback, the 25% lot coverage rule, the parking requirement and the architectural matching rules all belong to Class 2 and none of them reaches a Class 1 unit.

The ordinance says the combination out loud, which many do not: a unit approved under § 5-2P-5:A.1 “may be combined with an ADU approved under subsection 5-2P-5:A.2.” On a lot with an existing multifamily building the arithmetic is different and larger — conversion units in non-livable space up to 25% of the existing unit count, plus up to eight detached ADUs capped at the number of existing units (§ 5-2P-5:A.3–A.4, codifying SB 1211).

Owner-occupancy

You do not have to live there. Section 5-2P-6:G.1 states that ADUs are not subject to an owner-occupancy requirement, which is what Gov. Code § 66315 requires. Junior ADUs are different, and Hidden Hills has already written the current rule into its code rather than carrying the older, broader version most cities still show: owner occupancy applies, except where the junior ADU has its own sanitation facilities, or where the property is owned by a governmental agency, land trust or housing organization (§ 5-2P-6:G.2). The separate-bathroom exception is AB 1154, effective January 1, 2026, amending Gov. Code § 66333(b).

What an ADU costs in Hidden Hills (2026)

Our Signature plans are fixed-price — architectural design and full permit processing included, the same number whether the lot is in Hidden Hills, Calabasas or the Westside. Construction is separate: once your plans are permitted, you build it yourself, bring your own licensed contractor, or build with one of our vetted construction partners. The figures below are the estimated cost to build each model with an independent licensed general contractor.

Only the six single-story plans appear here. The three two-story Signature Homes are detached buildings of roughly 25 feet and cannot be permitted on a Hidden Hills lot as drawn — a detached ADU is limited to one story by § 5-2P-7:I — so quoting them on this page would be quoting something you cannot build.

What is fixed is the plan: our Signature plan fees run $7,490 to $12,990 — architectural design and full permit processing included. We can price it because the Signature Homes were engineered to pass objective ADU standards on paper, and the Hidden Hills standards are objective throughout Article P. The build figures above are an estimate by an independent licensed general contractor: your plans, your choice of builder.

For sizing and payback math, try our ADU calculator and the ROI calculator.

Development fees

Two square-foot thresholds do most of the work, and both come from state law before they come from the ordinance. An ADU under 750 sqft and any junior ADU are exempt from impact fees entirely — defined at § 5-2P-8:A.1 as Mitigation Fee Act and Quimby Act fees, expressly excluding water and sewer connection fees and capacity charges. A unit under 500 sqft does not increase assessable space and so escapes school fees (§ 5-2P-8:A.2; Gov. Code § 66311.5(c)(3); Education Code § 17620) — note that the exemption reaches a unit of less than 500 sqft, so at exactly 500 it no longer applies. Above 750 square feet, § 5-2P-8:A.3 requires any impact fee to be prorated to the ADU’s share of the primary dwelling’s floor area — and the ordinance states that formula the right way round, which is not universal in LA County.

The City has no ADU-specific permit fee. Hidden Hills’ schedule of fees and charges for land use and development services was adopted by Resolution No. 877 on November 10, 2014, and contains no accessory dwelling unit line item. Building permit and plan-check fees are valuation-based: the permit fee follows a tiered table against the ICC Building Valuation Data, plan review is charged at 85% of the permit fee, and three surcharges apply on top — 10% where California energy-conservation compliance is required, 5% for the Green Building Code and 5% for disabled access. Planning fees that may attach include Site Plan Review at $759 and, where a ridgeline is involved, a Ridgeline Permit Application at $2,741. Your Backyard Review includes a line-item estimate of the pass-through costs for your specific lot.

What it returns: renting your ADU

A Hidden Hills ADU is a long-term rental asset or it is family housing. Rentals of fewer than 30 days are barred outright by § 5-2P-6:D, regardless of when the unit was created, and state law goes one day further. Gov. Code § 66323(e) requires a term longer than 30 days for the state-mandated units, and § 66333(g) the same for a junior ADU, so a tenancy of exactly 30 days is allowed by the local text and barred by the statute. Write the lease for 31 days or more.

The trusted benchmark is HUD’s Fair Market Rent for the Los Angeles–Long Beach–Glendale metro area. Read it as a floor rather than a forecast, and in Hidden Hills read it as a low floor: the figure is calculated across all of Los Angeles County and every age and class of housing stock, so a new detached unit on an acre lot in a gated city is not the property it describes.

HUD Fair Market Rents for the Los Angeles–Long Beach–Glendale HUD Metro FMR Area, FY 2026 as revised, effective May 21, 2026 (91 FR 21301). HUD’s FY 2027 figures take effect October 1, 2026. Treat these as a floor rather than a forecast: a new, fully permitted detached ADU typically rents well above the metro benchmark, because the comparison pool includes much older and smaller stock from across the whole county.

Why the Hidden Hills market rewards the investment

Hidden Hills is the smallest kind of market and one of the strongest per lot. The city is roughly 1,700 residents on acre-plus parcels with a one-acre minimum lot size and a 150-foot minimum average width (§ 5-2C-2), which means nearly every property has the physical room for a 1,000 square foot detached unit without touching the house. That is unusual. In most of Los Angeles County the binding constraint on an ADU is land; here it is the 25% lot-coverage ceiling and the design-matching rules, both of which are solvable on paper before anyone digs.

The uses follow the market rather than the rent table. Multigenerational housing, a guest house that is legally a dwelling rather than a permanently non-conforming “pool house,” staff accommodation, a home office or studio detached from the main house — these are the briefs that come up on an estate lot, and all of them are better served by a permitted ADU than by an accessory structure that can never contain a kitchen. Where the property already carries an unpermitted guest unit built before January 1, 2020, § 5-2P-9:B provides a legalization path the City may not refuse on the ground that the unit fails current building standards or Article P itself. State law adds two protections the local text does not spell out: Gov. Code § 66311.7(e) bars impact fees and connection or capacity charges on a legalization permit, except where utility work is needed to meet Health & Safety Code § 17920.3, and § 66311.7(f) bars penalizing you for the unpermitted unit.

When an ADU isn’t the right move

Four situations where the honest answer in Hidden Hills is wait, change the plan, or do something else.

The rules that make Hidden Hills different

This is the part of the page that does not exist anywhere else, so it is worth being precise about what is settled and what is not. Everything below is sourced to HHMC Article P, HHMC Article K, the City’s own published ADU process, Gov. Code §§ 66314–66323, and Civ. Code § 4751.

The approval Article P never mentions

Hidden Hills is governed by two bodies. The City of Hidden Hills is the permitting authority. The Hidden Hills Community Association is a private homeowners association that owns and maintains the streets and infrastructure inside the gates and runs an Architectural Committee. Most residential land in the city is subject to HHCA covenants, conditions and restrictions recorded across eighteen separate sets of deed restrictions since 1950, and every resident pays an annual association assessment — facts the City states in its own Housing Element filed with HCD, along with the observation that the Association “has an independently elected board, outside the control of the City of Hidden Hills” and that “[t]he City has no authority to modify or remove the restrictions.”

None of that appears in Article P. It appears in the City’s published ADU process, which states that a “[c]ompleted application includes HHCA Architectural Committee approval” and that the 30-day decision clock for a preapproved plan “begins upon submittal of the completed application to the City.” The homeowner instructions on the same page list obtaining Architectural Committee approval as step three of four, ahead of the building permit. The City’s fee schedule reinforces it from the other direction: there is a $2,580 Site Plan Review Hearing fee charged specifically where an applicant comes forward without HHCA approval.

The practical advice follows from the split. Treat the Association review as a real project requirement, because it is one, and start it early. Treat the City’s statutory clock as a separate thing that runs on its own terms. If the City declines to accept an application as complete solely for want of Association sign-off, that is the moment to ask, politely and in writing, which provision of Article P the application fails.

Ridgeline review versus ministerial approval

Hidden Hills sits in the Santa Monica Mountains foothills, and HHMC Article K regulates building near ridgelines with unusual reach. Section 5-2K-3:A provides that no new structure or addition “in any zone” may be placed on a ridgeline, or within 50 vertical feet or 50 horizontal feet of one, or positioned so it appears silhouetted against the sky from a street, except as part of a development project approved by the Planning Agency. Getting that approval means a Ridgeline Development Permit: a contoured plot plan, a visual analysis, a noticed public hearing with mailed notice to every owner within 500 feet, and five discretionary findings the Planning Agency must make before it can approve (§ 5-2K-4). The application fee is $2,741. A ridgeline is defined broadly at § 5-2K-2:B — visual dominance, a silhouetting appearance, or prominence from existing streets.

The exemption at § 5-2K-5 does not help a new ADU. It reaches only repair, replacement or remodelling of a residence or accessory structure that was legally established as of September 1, 2011, in substantially the same location, with cumulative enlargement capped at 25% or 1,200 square feet, whichever is less. A newly built detached ADU on an undeveloped part of the lot is not that.

Wildfire zones, sprinklers and what they do not do

Hidden Hills is usually described as a Very High Fire Hazard Severity Zone city, and that shorthand is close but not exact. On the 2025 state maps the City’s own notice states that Hidden Hills “contains areas designated as Moderate, High, and Very High Fire Hazard Severity Zones” — all three tiers are mapped inside the city, so the tier that governs your build is a parcel-level fact rather than a citywide one. The whole city is Local Responsibility Area, which means the City and Los Angeles County Fire administer the standards rather than CAL FIRE.

What this does not do is stop you building. There is no categorical wildfire prohibition on ADUs in Hidden Hills, and the ordinance handles sprinklers in the homeowner’s favor: sprinklers are required in an ADU or junior ADU only if they are required in the primary residence, and building an ADU does not trigger a sprinkler retrofit in the existing house (§ 5-2P-6:C; Gov. Code § 66314(d)(12)). Fire hazard mapping bears on construction assemblies, defensible space and access rather than on entitlement. Confirm the tier for your specific parcel rather than assuming the citywide headline applies to it.

Where the local text and state law diverge

Article P is unusually clean — it already carries AB 1154, SB 1211 and the SB 543 completeness clock. Three divergences are worth knowing about anyway.

Three internal drafting errors in Ord. 407 are worth flagging if you or your designer are reading the code directly, because each sends you to a provision that does not say what the cross-reference implies. Section 5-2P-6:A.1 describes a “Class 1 ADU approved under subsection 5-2P-5:B” when Class 1 units are approved under § 5-2P-5:A; § 5-2P-6:A.2 refers to “5-2P-5:8” where it means 5-2P-5:B; and § 5-2P-6:I twice cites a “subsection 5-2P-6:L.2” that does not exist, since § 5-2P-6 runs only A through J. Section 5-2P-8:B.3 likewise refers to “subsection (h)(2)(A)” and “(h)(2)(B),” lettering carried over from the state statute the section was adapted from. None of these changes the substance, and none is a reason to doubt the ordinance — but citing them back to the City verbatim will confuse the conversation.

Recent state-law changes and where Hidden Hills stands

Because Ord. 407 was adopted in January 2026, it incorporates amendments that most LA County ordinances are still behind on.