Building an ADU in Hermosa Beach. Rules, costs, timeline.

Can I build an ADU in Hermosa Beach?

Five things to know before you start

If you read nothing else here, read these. They are the five points that change the most Hermosa Beach projects.

How many units your lot allows

HBMC § 17.21.050(A) writes the Gov. Code § 66323 stack into local law and then does the thing most ordinances leave ambiguous: it states outright that a unit approved under (A)(1) may be combined with one approved under (A)(2). On a single-family lot that means:

Three accessory units, then, on top of the house already standing. Nothing in the chapter caps you at “one ADU plus one junior ADU,” which is what most homeowners have been told.

There may be a fourth. HCD’s ADU Handbook (March 2026, p. 18) closes its table of allowable combinations with this: in addition to the units described in Gov. Code § 66323, a local agency must allow at least one unit described in § 66314, and may impose objective standards on it. That is the local track. HCD made the same point to Hermosa Beach directly in its December 2025 findings letter, telling the City that § 66314 units had to be reflected in the ordinance “to enable maximum unit allowances.” The City answered by adding the Class 2 track — but Chapter 17.21 does not say in terms that the two tracks stack. We would treat four as the state floor under Gov. Code § 66316 and confirm the as-applied answer with Planning before designing to it.

On a multifamily lot the numbers are larger. § 17.21.050(A)(3) allows at least one conversion ADU in non-livable space such as a storage room, boiler room, attic, basement or garage, up to 25 percent of the existing unit count. § 17.21.050(A)(4) allows up to eight detached ADUs on a lot with an existing multifamily building, capped at the number of existing units (Gov. Code § 66323(a)(4)(A)(ii)), or two on a lot with a proposed one. Those two may be combined as well, and the chapter says so expressly (§ 17.21.050(A); Gov. Code § 66323(a)).

Owner-occupancy

You do not have to live on the property. HBMC § 17.21.060(G)(1) says ADUs are not subject to an owner-occupancy requirement, which is the local expression of Gov. Code § 66315. A junior ADU is treated differently, but narrowly. Owner-occupancy applies in general under § 17.21.060(G)(2)(a), and is waived under (G)(2)(b) in two situations: where the junior ADU has its own separate sanitation facilities, and where the property is entirely owned by a governmental agency, land trust or housing organization.

The sanitation carve-out is AB 1154, effective January 1, 2026, which narrowed Gov. Code § 66333(b) to shared-sanitation cases only. Most cities in the region have not amended their text to match. Hermosa Beach wrote it in. Give the junior ADU its own bathroom and the requirement does not reach you. A junior ADU still records a deed restriction before its certificate of occupancy (§ 17.21.060(H)); an ADU records nothing.

What an ADU costs in Hermosa Beach (2026)

Our Signature plans are fixed-price — architectural design, structural engineering, Title 24 compliance and full permit processing included, the same number in Hermosa Beach as in every other city we serve. 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.

Development fees

Impact fees are the big exemption, and Hermosa Beach has written the state rule into § 17.21.080(A). No impact fee applies to a junior ADU, or to an ADU at or below the statutory 750-square-foot line. Above that, the fee is charged in proportion to the main house rather than at the full rate a new dwelling would pay, and § 17.21.080(A)(3) sets out the arithmetic: the floor area of the ADU, divided by the floor area of the main house, times the fee a new dwelling would be charged. A unit below 500 square feet is also outside school impact fees under Education Code § 17620 (§ 17.21.080(A)(2); Gov. Code § 66311.5(c)(3)).

What is not exempt: building-permit and plan-check fees, which are based on project valuation rather than on the fact that you are building an ADU, and utility connection fees and capacity charges, which § 17.21.080(A)(1) expressly carves out of the definition of an impact fee. Junior ADUs and statutory-track conversion ADUs on a single-family lot need no new or separate utility connection and no connection fee at all, unless they are built with a new house (§ 17.21.080(B)(2)). Other ADUs do, at a charge proportionate to the burden they place on the system (§ 17.21.080(B)(3)).

We have deliberately not published a dollar figure for Hermosa Beach permit and plan-check fees on this page. The City publishes its own schedule and it is valuation-based; your Backyard Review includes a line-item estimate of the pass-through costs for your specific lot, priced against the current schedule rather than a number we made up.

What it returns: renting the unit

Hermosa Beach is a 30-day-minimum city, so the asset you are building is a long-term tenancy rather than a nightly one. The benchmark we anchor on is HUD’s Fair Market Rent for the Los Angeles–Long Beach–Glendale metro area, FY 2026 as revised. It is a metro-wide floor across all housing stock and all ages, which is exactly why it is the right anchor: a new, fully permitted detached unit in a beach city rents at or above it, not below.

HUD FMR figures: FY 2026 Fair Market Rents, Los Angeles–Long Beach–Glendale HUD Metro FMR Area, as revised effective May 21, 2026 (Federal Register, 91 FR 21301, published April 21, 2026). These are metro-wide figures across all housing stock and ages, and they are presented as a floor rather than a forecast. We have not published Hermosa Beach-specific rent bands here because we do not have a citable source for them; for payback math keyed to your lot, your model and your financing assumptions, use the ADU ROI calculator.

Why the Hermosa Beach market rewards the build

The city covers 1.4 square miles and has effectively no buildable greenfield left, which is the whole argument. New supply arrives one accessory unit at a time, and the rules now allow three of them on a single-family lot. The 30-day floor removes nightly-rental supply from the market, which tightens the long-term rental pool that a permitted ADU competes in.

The South Bay employment base — aerospace and engineering in El Segundo and Hawthorne, and the office corridor along Rosecrans — keeps long-term tenant demand steady rather than seasonal. And a fully permitted unit adds recorded, comp-able square footage to a parcel in a market where turnover is low and comparables are scarce.

The regulatory picture helps too, and it is unusual enough to say plainly: this is a city whose ADU chapter the State has reviewed and signed off on. That is not true of most of the region. It means the text you are designing against is the text that will be applied, rather than a chapter waiting to be corrected under Gov. Code § 66326.

When an ADU isn’t the right move

Not every Hermosa Beach property is a good ADU candidate, and it is cheaper to learn that now than after you have paid for plans. Five situations where we would tell you to wait, choose a different unit type, or not build:

• Your numbers depend on nightly rental income. They will not work. HBMC § 17.21.060(D) bars renting an ADU or junior ADU for a term shorter than 30 days, whenever the unit was created, and Gov. Code § 66323(e) requires a term longer than 30 days on the statutory track. Run the math on a month-to-month tenancy or stop here.
• You want a two-story detached backyard house. It does not exist in this city. Sixteen feet, one story, no above-garage exception. If the second story is the point, the honest options are an attached unit built into the main house, or a different city.
• Your lot has almost nothing usable after setbacks. Hermosa Beach parcels are small. After 4-ft side and rear setbacks and a 25-ft front setback on the local track, some lots simply cannot take a free-standing building worth building. A garage conversion or a junior ADU usually returns more on those lots.
• You need a firm date and you are in the Coastal Zone. The coastal permit now carries a 60-day clock, which is a real improvement — but your ADU application is not complete until the coastal application is complete (§ 17.21.040(C)(6)(a)). Assembling that submittal properly takes time no builder can compress.
• You want to sell the unit on its own one day. Hermosa Beach has not opted in under AB 1033, so the condominium route at Gov. Code § 66342 is not available here. § 17.21.060(E) bars separate conveyance except under Gov. Code § 66341, which is a qualified-nonprofit route, not a homeowner exit.

The rules that make Hermosa Beach different

The Coastal Zone

Much of the city sits inside the California Coastal Zone established by the Coastal Act of 1976 (Pub. Resources Code § 30000 et seq.). The boundary runs inland from the beach and does not cover the whole city, so whether a given property is inside depends on the address. We confirm it against the City’s boundary map before anyone commits to a schedule.

HBMC § 17.21.040(C)(6) says that nothing in state ADU law or the local code lessens the effect of the Coastal Act. It then attaches three qualifications. A complete coastal application is required before the ADU application counts as complete. The coastal permit must be approved or denied within 60 days. And where the City has no certified local coastal plan or program, it must notify the Coastal Commission at once.

Layered on top is Gov. Code § 66329 as rewritten by AB 462: concurrent review, no public hearing, and no Coastal Commission appeal of a local decision. The timeline consequences are worked through in the timeline section.

Design rules on the local track — and how to avoid them

Hermosa Beach has some of the most specific architectural requirements in the South Bay, and every one of them lives in § 17.21.070, which applies only to local-track units. The exterior wall, roof, window and door materials and colors must match the main house. The roof slope must match its dominant slope. Exterior lighting is limited to down-lights. The unit needs an independent exterior entrance, interior dimensions of at least 10 feet in every direction, and a minimum interior wall height of 7 feet.

Two privacy rules go further than most cities. No window or door may have a direct line of sight to an adjoining residential property — each one must either be placed where no such line exists or be screened with fencing, landscaping or privacy glass. And any window or door within 30 feet of a property line that is not a public right-of-way must be clerestory, with the bottom of the glass at least six feet above the finished floor, or use frosted or obscure glass (§ 17.21.070(H)(6)–(7)). Section 17.21.070(I) then requires drought-tolerant evergreen screening between the ADU and adjacent parcels, from the City’s approved plant list.

All of it falls away on the statutory track. Both subsections carry an express savings clause — §§ 17.21.070(H)(8) and (I)(5) state that the requirements “shall not preclude any unit subject to Government Code section 66323” — and Gov. Code § 66323(b) independently bars a city from imposing objective design standards on a statutory unit. That is why the size decision and the design decision are really one decision here.

What the State said about this ordinance

Most city ADU chapters in the region have never been through a state compliance review. This one has, twice, and the second letter closed the file. HCD wrote to the City on December 29, 2025 with written findings under Gov. Code § 66326(a) against the prior Ordinance No. 1484, giving the City 30 days to respond. Ord. No. 26-1498 is the answer. Here is what changed:

HCD reviewed the adopted ordinance on April 14, 2026 and wrote to the City on May 8, 2026 that it “is substantially compliant with State ADU Law,” and that the Department “considers the matter of compliance with Government Code sections 66310-66342 closed.” Both letters are linked in the sources section.

What that means for you is practical rather than ceremonial. When a local chapter is out of step with the statute, a homeowner has to argue preemption at the counter, and Gov. Code § 66316 makes a noncompliant ordinance null and void until it is fixed. Here there is almost nothing to argue about. We found exactly one place where the local text reads narrower than the statute — the impact-fee cutoff at exactly 750 square feet, covered in the costs section — and it affects one square foot of one threshold.

Short-term rentals and separate sale

Two rules close doors, and both are worth knowing before you model returns. HBMC § 17.21.060(D) bars renting an ADU or junior ADU for a term shorter than 30 days, and applies that regardless of when the unit was created — there is no legacy carve-out. State law agrees rather than preempting: Gov. Code § 66323(e) requires a term longer than 30 days on the statutory track, and § 66333(g) says the same for a junior ADU, so a month-to-month tenancy is the safe floor.

On separate sale, Hermosa Beach has not opted in under AB 1033, so the condominium route at Gov. Code § 66342 is closed here. § 17.21.060(E) bars conveying an ADU or junior ADU separately from the lot and the main dwelling, with one carve-out: “as otherwise provided in Government Code section 66341.” That is the AB 587 route, under which a qualified nonprofit that built the unit may convey it to a qualified low- or moderate-income buyer under a recorded tenancy-in-common agreement. It is mandatory statewide and needs no opt-in — and it is not a homeowner exit. The two statutes are different things, and pages that write “§§ 66341–66342” as one citation get this wrong.