Building an ADU in Downey. Rules, costs, timeline.

Yes, and Downey is one of the easier cities in Los Angeles County to do it in. The city issued 244 ADU building permits in 2025, up from 202 the year before, and more than every city in the county except Los Angeles, Glendale, Long Beach and Lakewood. It rewrote its ADU rules twice in fourteen months to keep them current with state law — most recently in Ordinance 26-1544, adopted June 9, 2026. ADUs are allowed in the R-1, R-2, R-3, R-3-O and M-U zones, and more broadly anywhere single-family or multifamily residential use is allowed. Junior ADUs are the one exception: they are not allowed on multifamily-zoned property.

Two ADUs and a junior ADU, and this is written into the Downey code rather than being something you have to argue for from the statute. Section 9414.08(a)(1) says "a maximum of two ADUs can be permitted on a single-family lot," made up of one attached or detached new-construction ADU plus one ADU converted from existing space. On top of that, subsection (a)(2) allows one junior ADU, which must be converted from existing space inside the house and is capped at 500 square feet of interior livable space. So a Downey single-family lot can carry the house plus three additional units. Multifamily lots run on a separate section, § 9414.10, and it is more generous still: on a lot with an existing multifamily building you may add up to eight detached ADUs, capped at the number of units already there, and separately convert non-livable space — storage rooms, boiler rooms, attics, basements, common areas, garages — into ADUs up to 25 percent of the existing unit count, with at least one always allowed. A lot with a proposed multifamily building gets two. Junior ADUs are the exception in the other direction: they are not permitted on multifamily-zoned property. Most cities we cover write "one" and leave you to cite state law for more.

For a detached or attached ADU, 850 square feet for a studio or one-bedroom and 1,000 square feet for a unit with two or more bedrooms. An attached ADU carries one more test: it is limited to 800 square feet or 50 percent of the floor area of the existing primary dwelling, whichever is more, capped at 1,000. Read that "whichever is more" carefully, because it works in your favor — on a small house the 800 square feet applies rather than the percentage. Conversions are treated differently again: an ADU converted from existing permitted space is exempt from the maximum sizes altogether, and may expand by up to 150 square feet beyond the existing structure for ingress and egress.

Not at 800 square feet or under. Downey Municipal Code § 9414.08(b)(3) exempts a new ADU of 800 square feet or less from floor area ratio, lot coverage, design standards and open space requirements, and (b)(4) says an ADU of 800 square feet is permitted "without the applicability of development standards of the underlying zone, such as FAR, lot coverage, front setbacks, design standards, and open-space requirements." Note front setbacks in that list. Above 800 square feet those standards come back into play, which is the real decision point on a tight Downey lot: an 850 square foot unit is subject to rules an 800 square foot unit is not. Two details that catch people out: patio covers and similar covered structures do not count toward the ADU size but do count toward your lot coverage, and an attic or loft over five feet in height counts as livable area.

Four feet to the rear and side property lines for any new attached or detached ADU, including one built above an existing or proposed structure. Front setbacks are the interesting part in Downey and are covered in the next answer. Two provisions work in your favor on an older lot. A conversion of a permitted structure, or a new structure built in the same location and to the same dimensions as an existing one, may keep the existing setback as long as building code requirements are met. And an unpermitted accessory structure may keep its legal nonconforming setbacks as part of a conversion or addition, provided it does not present a health and safety threat and does not violate a recorded easement.

Sometimes, and Downey is the only city we cover that writes this down. First check whether the rule reaches you at all: § 9414.12 exempts ADUs and JADUs created under Gov. Code § 66323 from front setbacks, and § 9414.08(b)(4) permits an 800 square foot ADU without the underlying zone's development standards, front setbacks expressly among them. If you are on either route you do not need any of what follows. For the larger ordinance ADU the general rule is no — § 9414.08(c)(2) says a new ADU or JADU cannot sit in the front setback area of the underlying zone. But there is an exception at (c)(2)(i): if it is physically infeasible to build an 800 square foot ADU anywhere else on the lot, an 800 square foot ADU that complies with all other development standards is allowed to encroach into the front yard setback. It may not encroach into the driveway for on-site parking or garage access. The ordinance defines "infeasible" precisely, which is what makes it usable: the lot area, minus existing structures, the front, side and rear setbacks, easements, a pool and the required driveway, must not leave enough room for a continuous 800 square foot area. You submit architectural plans and exhibits demonstrating it, a utility provider letter if utility standards further restrict the lot, and the city inspects to verify.

A detached ADU on a lot with an existing or proposed single-family dwelling gets 16 feet. That rises to 18 feet if the lot is within a half-mile walking distance of a major transit stop or a high-quality transit corridor, and you may add two more feet — 20 feet total — to carry a roof pitch that matches the main house. An ADU attached to the house gets 25 feet, or the underlying zone's height limit if that is lower. Nothing under this section may exceed two stories. One design restriction that surprises people: balconies and decks are not permitted on the second floor of an ADU, and all windows and exterior glass doors in a second-story ADU must be obscured or frosted glass.

This is the most Downey-specific thing on the page, and it is worth reading before you pick a design. Section 9414.12 sets objective design standards to preserve the property's existing architectural features. The ordinance reads: "The architectural design features of an ADU including but not limited to finish materials, colors, roof forms, windows and doors shall match those of the primary dwelling." The roof slope must match the dominant roof slope of the main house — with one exception worth knowing, at § 9414.12(f)(1): a unit converted from existing space that has not been demolished and replaced in the same location does not have to match. The primary entrance must have a covered front entrance whose roof form and slope match both the ADU and the primary dwelling, and it must be oriented toward the street. The three exceptions are narrower than they sound: the unit is at the rear of the property and attached to the house in a way that makes a street-facing entrance physically infeasible; the unit is on the second floor of an existing primary dwelling; or the unit is not visible from the public right of way. No dwelling may have its front entrance facing the rear of the property or a public alley. ADUs larger than 500 square feet need washer and dryer connections. Only one kitchen is allowed per dwelling. Front entries need downward porch lighting, and staircases must be within enclosed interior space. Where you do have an exterior staircase, it keeps the four-foot side and rear setback, may never sit in a front setback, and may not turn most of its face toward the front yard.

Yes, and the city wrote it into the same section. Section 9414.12 ends by saying that ADUs and JADUs created pursuant to Government Code § 66323 "are not subject to lot coverage, front setbacks, and design standards but must comply with building code and health and safety requirements for dwellings." A § 66323 unit means a conversion ADU inside existing space, a junior ADU, or the by-right detached unit. So the matching materials, the matching roof slope and the street-facing entrance are constraints on the larger ordinance ADU and simply do not reach the state route. Separately, a new ADU of 800 square feet or less is exempt from design standards under § 9414.08(b)(3). That is a real design decision rather than a technicality: if you want a contemporary ADU behind a 1950s Downey ranch house, the size you choose determines whether you may have it.

One uncovered off-street space per ADU, and the way Downey lets you provide it is unusually practical: the space may sit on the existing legal driveway, including in setback areas, or as tandem parking. So on most Downey lots the driveway you already have satisfies it. Six exemptions remove the requirement entirely, and one of them swallows most projects: § 9414.14(a)(1)(iii) exempts an ADU that is "part of the proposed or existing primary residence or an accessory structure" — which is every conversion, including a garage conversion. The others are a half-mile walking distance to public transit, an architecturally and historically significant historic district, on-street parking permits that are required but not offered to the ADU's occupant, a car share vehicle stop within one block, and an ADU applied for alongside a new primary dwelling that meets one of the other criteria. A junior ADU converted from an existing attached garage requires no additional parking. And if you are converting a garage, state law is on your side twice over: Gov. Code § 66314(d)(11) bars the city from making you replace the parking the garage was providing. Note that a proposed new primary dwelling is not exempt and must meet its own parking requirement.

Only if the main house has them. Downey Municipal Code § 9414.14(b) requires fire sprinklers in an ADU if sprinklers are required in the primary dwelling, which tracks state law. On Downey's housing stock, much of it built before residential sprinkler requirements, that usually means no. And building the ADU cannot create the requirement: § 9414.14(b)(2) says "the construction of an ADU does not trigger a requirement for fire sprinklers to be installed in the existing primary dwelling." Subsection (b)(3) adds that a JADU is not treated as a separate dwelling for fire or life-safety purposes. So a whole-house retrofit is not something you need to budget against.

Building permit and plan check fees are valuation-based, so they scale with the value of the work and are better quoted against real drawings than guessed from a table. What state and local law take away is worth knowing precisely. No impact fee may be charged for an ADU with 750 square feet of interior livable space or less, or a junior ADU with 500 square feet or less — Downey's § 9414.18(a)(1) uses exactly that wording, matching Gov. Code § 66311.5(c)(1). Above 750 square feet, any impact fee must be charged proportionately, comparing the ADU's square footage to the primary dwelling's. Note the carve-out in the same subsection: "impact fee" there does not include a connection fee or capacity charge for water or sewer service. But read § 9414.18(b) before you budget for one. Subsection (b)(2) says a converted ADU, a junior ADU, or an ADU allowed under Gov. Code § 66323 is not required to have a new or separate utility connection "nor is a connection fee or capacity charge required." Where a connection fee is chargeable — a new detached ADU, or an ADU built alongside a new house — (b)(4) requires it to be proportionate to the burden the unit creates, measured by floor area or drainage-fixture units, and it may not exceed the reasonable cost of providing the service. School facilities fees are a separate regime from outside the ordinance: Education Code § 17620(a)(1)(C) lets a district levy them only where the increase in assessable space exceeds 500 square feet.

Budget six to twelve months from first drawing to permit in hand, then construction on top. The statutory review is 60 days from a complete application and it is ministerial — a checklist, not a judgment call, with no hearing and no neighbors to win over. Gov. Code § 66317 sets that clock and it binds the permitting agency as a whole, so any internal referral sits inside the 60 days rather than in front of them. If the city misses the deadline without acting, the application is deemed approved. Two things stop that clock, both at § 9414.16(b) and mirrored in Gov. Code § 66317(a)(3): a delay you request, and an ADU filed together with a permit application for a new primary dwelling, in which case the city may hold the ADU until it acts on the house. Before the clock starts, the city has 15 business days to determine completeness in writing (§ 66317(a)(2)(A)), the notice must list every incomplete item and say how to cure it ((a)(2)(B)), and the city may not later require anything it left off that list ((a)(2)(D)). Miss the 15-day deadline entirely and the application is deemed complete ((a)(2)(F)).

Yes. The Building and Safety Division publishes a pre-approved detached ADU program, with minimum submittal requirements and a submittal checklist among its handouts, implementing the state requirement that cities make pre-approved plans available. We have not seen the current plan set, so we are not going to tell you what is in it or how it compares on cost — ask Building and Safety what is on the shelf. Worth knowing that the city's own handouts page carries a warning that not all of its handouts reflect current codes, which is a good reason to confirm anything you download against the ordinance date.

Downey runs across three ZIP codes and the spread is real but modest, about 15 percent. In June 2026 the Zillow home value index was roughly $952,000 in 90240, $932,000 in 90241 and $828,000 in 90242, with rent indices of about $2,900, $2,300 and $2,250 respectively. Check which ZIP your own address carries before taking a blended Downey figure seriously. We are not going to publish a bedroom-level asking rent, because the sources we trust are either blended across all unit sizes or are voucher standards rather than market rents. Start from HUD's Small Area Fair Market Rents for your ZIP, then check live listings nearby.