ADU rules in Los Alamos County

A detached backyard ADU is allowed without a hearing, but only in seven residential zone districts: RA, RE, SFR-1, SFR-2, SFR-3, RM (RM-1 and RM-2) and MFR-L. Table 26 marks the Accessory Dwelling Unit row 'A' in those columns and leaves SFR-4, SFR-5, MHC, MFR-M, MFR-H and every mixed-use, non-residential and open-space district blank, which Sec. 16-12 says means the use is prohibited there; note 1 to the table spells out that ADUs are permitted in SFR-1, SFR-2 and SFR-3 and prohibited in SFR-4 and SFR-5. The ADU-eligible single-family districts are the larger-lot ones (minimum lot area 13,000 sq ft in SFR-1, 12,000 in SFR-2, 10,000 in SFR-3), so a great many Los Alamos townsite and White Rock lots zoned SFR-4 (8,000 sq ft) or SFR-5 (5,000 sq ft) cannot have one at all. Where allowed, a detached unit must stand at least ten feet from the primary dwelling and is held to the district's accessory-structure height limit: 15 feet in the SFR and RM districts, 20 feet in RA and RE, 12 feet in MFR-L. It also counts against the per-lot accessory-structure cap, which is three structures in the SFR and MFR-L districts, four in RM and five in RA and RE.

Attached, internal and detached ADUs are the same use in Los Alamos County and share exactly the same zone gate, so an addition or a basement or interior conversion is available in RA, RE, SFR-1, SFR-2, SFR-3, RM and MFR-L and nowhere else. The Development Code defines an accessory dwelling unit as one that 'may be attached to the primary dwelling, contained within the primary dwelling, or built as a detached accessory structure and must contain a separate kitchen', and Sec. 16-18(a)(2) allows 'no more than one accessory dwelling unit either attached or detached per lot'. The same 250 to 800 sq ft window, the same one-parking-space rule and the same no-separate-sale rule apply. Because the definition ties an ADU to 'a primary single-family or two-family detached dwelling', an ADU is not available next to a townhouse, triplex, fourplex or apartment building even in RM and MFR-L where the row is marked 'A'. One practical difference: an attached or internal unit is part of the main building rather than an accessory structure, so it needs the building permit but not the separate accessory structure permit a detached unit requires.

Sec. 16-18(a)(4) sets both a floor and a ceiling: the ADU 'shall not be less than 250 square feet or more than 800 square feet', and its square footage 'shall not exceed the gross floor area of the primary dwelling', so on a small primary house the effective cap is the house's own size. The 250 sq ft minimum, and the sentence requiring compliance with building-code minimum room sizes and ceiling heights, were added by Code Ordinance No. 02-370 adopted 10-7-2025; before that the rule was simply an 800 sq ft ceiling. Underlying district standards stack on top: lot area, setbacks and lot coverage maxima (30 percent in SFR-1 and SFR-2, 35 percent in SFR-3, 40 percent in RM) all apply, and a detached unit is capped at the district's accessory-structure height, 15 feet in SFR and RM, 20 feet in RA and RE, 12 feet in MFR-L. None of these numbers is negotiable: Sec. 16-18(a)(8) provides that 'No variances or administrative deviations shall be granted for the standards outlined in this section.'

Nothing in the Los Alamos County Development Code conditions an ADU on the owner living on the property, at application or afterward. Sec. 16-18(a) lists eight standards for accessory dwelling units (building permit, one per lot, underlying dimensional standards, the 250 to 800 sq ft size window, ten-foot separation from the primary dwelling, one off-street parking space, no separate sale, and no variances) and none of them mentions occupancy by the owner; a full-text search of the whole county code turns up no owner-occupancy rule touching ADUs. The contrast within the same article is deliberate: a bed and breakfast, the county's listed short-stay lodging use in residential districts, 'shall be owner-occupied or shall be occupied by a resident manager' under Sec. 16-16(b)(2). New Mexico has no statewide ADU statute, so no state preemption is doing any work here in either direction.

Sec. 16-18(a)(6) requires the ADU to 'provide one dedicated off-street parking space', and Table 27 in Sec. 16-28(c) agrees, listing 'Accessory dwelling: 1 space per unit'. The space has to sit in a designated parking area such as a driveway or garage, no more than one tandem space may be counted, and parking on lawns or sidewalks is prohibited outright. That space is in addition to the primary dwelling's own requirement, which is two spaces for a single-family home, so a typical SFR lot with an ADU has to show three off-street spaces on the site plan. The parking reductions in Sec. 16-30 (shared parking under Table 29 and the additional reductions under Table 30) are written only for the DTLA, WRTC, mixed-use and non-residential districts, so they do not reach a residential ADU, and Sec. 16-18(a)(8) bars variances and administrative deviations from the ADU standards.

An ADU is an allowed accessory use rather than a discretionary one. Sec. 16-12 explains that an 'A' in Table 26 means the use 'is permitted in the respective zone district as an accessory use', with no conditional use permit and no Planning and Zoning Commission hearing. Every ADU needs a building permit before construction begins under Sec. 16-18(a)(1), and because a detached unit is an accessory structure it also needs an accessory structure permit under Sec. 16-73(c), a section that names 'accessory dwelling units (ADUs)' expressly; that permit is issued by the community development director or their designee and 'shall be approved if it complies with all applicable standards in this Code, any other adopted county regulations'. Table 49, the procedures summary table, lists the accessory structure permit under Administrative Decisions with the director deciding and the Commission hearing appeals, and requires no published, mailed or posted notice, no pre-application meeting and no neighborhood meeting. Los Alamos County operates its own building official under Chapter 10, Article II, so the building permit is county-issued rather than handled by the state Construction Industries Division, and Sec. 16-96 requires the director's written endorsement of zoning compliance before any building permit issues.

The Development Code places no restriction on renting an ADU for ordinary long-term tenancy: there is no owner-occupancy rule, no minimum lease term, no cap on rental ADUs and no rental registration anywhere in Chapter 16. What is barred is splitting the two homes apart: Sec. 16-18(a)(7) provides the ADU 'shall not be subdivided from the primary dwelling or sold under separate ownership from the primary dwelling' unless it independently meets all applicable requirements for a primary dwelling and all dimensional standards for a separate lot. Short-term rental sits in a genuine gap. Article VI defines 'short term rental' as a residential dwelling rented 'for the purpose of transient lodging for a period of time not to exceed 30 consecutive days', but no short term rental row appears anywhere in Table 26, which makes it an unlisted use the community development director must classify under Sec. 16-13; the only listed short-stay lodging use allowed in residential districts is a bed and breakfast, which requires a conditional use permit, must be owner-occupied or occupied by a resident manager, and is capped at five guestrooms and 30 consecutive days per guest. The county has no short-term rental licensing ordinance; the only county-wide short-stay layer is the Lodgers' Tax in Chapter 36, Article II, a 3 percent plus 2 percent occupancy tax on lodging of under 30 consecutive days with a vendor's license requirement, which does not apply where 'the taxable premises does not have at least three rooms or three other units of accommodations for lodging'.