ADU rules in Doña Ana County

The Unified Development Code lists 'Accessory dwelling' as its own residential use in Table 5.1, and whether you can build one turns entirely on your zone. It is permitted by right (P) in the transect zones T2 (Rural), T3 (Neighborhood Edge), T4 (General Neighborhood) and T5 (Town Center) and in D2 and D2L (Medium Density Residential). It requires a special use permit (S) in R5 and R5L (Rural Density Residential, 5-acre minimum lots) and in D1 and D1L (Low Density Residential), which together cover most of the county's low-density unincorporated land. It is not listed at all, and so is not allowed, in the N (Natural), D3 (High Density Residential), DM (Mobile Home Park), MU (Mixed Use), commercial (C1 to C3) and industrial (I1 to I3) zones. A detached unit is an accessory building, so § 350-502B(10) bars it unless a principal building is permitted and already exists on the lot.

The county draws no line between detached and attached: § 350-702 defines an accessory dwelling as 'a separate, complete unit with a separate entrance, kitchen, sleeping area, and full bathroom facilities, which is either attached or detached from the primary dwelling on the lot.' The same Table 5.1 row therefore governs an addition, a converted basement or an internal conversion, so the identical zone split applies (by right in T2, T3, T4, T5, D2 and D2L; special use permit in R5, R5L, D1 and D1L; not allowed in D3, DM, MU and the commercial and industrial zones). Article III's community-type track confirms internal units count, capping 'the habitable area of an accessory dwelling within the principal building or of an accessory building' at § 350-304F(2). The site standards tables set setbacks and height separately for principal and accessory buildings, so an attached or internal unit follows the principal-building rows while a detached casita follows the accessory-building rows.

The caps live in the per-zone Site Standards tables under § 350-502, not in a single ADU section. Table 5.4 (T2, R5 and R5L) reads '1 accessory dwelling limited to 1,800 square feet and 1 accessory dwelling permitted for every 5 acres of lot area,' Table 5.5 (T3) '1 accessory dwelling permitted per lot limited to 1,800 square feet,' Table 5.8 (D1 and D1L) '1 accessory dwelling limited to 1,800 square feet and permitted by special use permit,' and Table 5.9 (D2 and D2L) '1 accessory dwelling limited to 800 square feet.' Tables 5.6 (T4) and 5.7 (T5) say only '1 accessory dwelling permitted per lot' with no floor-area figure, so there the binding limits are lot coverage (65% in T4, 80% in T5) and setbacks. Height is capped separately: accessory buildings may be 2 stories in T2 through T5 and 35 feet in D1, D1L, D2 and D2L, and in R5, R5L, D1, D1L, D2 and D2L an accessory building may not exceed 5,000 square feet or 10% of the lot area, whichever is less.

Nothing in the UDC requires the owner to live on the property. A full-text read of Article V (the use matrix and all site standards), Article II (including the § 350-204D special use permit decisional criteria) and the Article VII glossary on 2026-08-04 turned up no owner-occupancy, primary-residence or family-relationship condition attached to an accessory dwelling. The county writes such a condition explicitly when it wants one: the glossary's bed-and-breakfast definition says 'the owner resides on the lot' and the inn definition says 'the owner or resident manager resides on the lot,' language that appears nowhere in the accessory dwelling provisions. New Mexico has no statewide ADU statute that would preempt the question either way.

The UDC sets no numeric off-street parking minimum for houses or accessory dwellings. § 350-603G states that 'parking requirements for multifamily and all nonresidential uses shall follow the Institute of Transportation Engineers (ITE) Parking Generation Manual, most current edition,' and § 350-502F(4) likewise limits space counts to multifamily and nonresidential land uses; single-family and accessory dwellings are simply not covered. What does apply is where parking may sit: the Site Standards tables set parking and storage setbacks (in T2, R5 and R5L parking must be 20 feet behind the front facade; T3 uses 15 feet), and § 350-502F(2) lets parking in use zones occupy no more than a third of the area between the principal building and the primary frontage line. If the unit is short-term rented, Chapter 280 § 280-4F separately requires on-site off-street parking via a garage, parking shelter or driveway.

It depends on the zone. Where Table 5.1 marks the use 'P' (T2, T3, T4, T5, D2, D2L) the review is administrative: a detailed site plan under § 350-208A ('a detailed site plan shall be required for all uses and buildings,' with a licensed professional needed only for commercial, industrial and multifamily work) and a building permit from the county's own Building Services Division under § 350-210A. Doña Ana County runs a certified building department, so building permits are county-issued rather than issued by the state Construction Industries Division. Where Table 5.1 marks 'S' (R5, R5L, D1, D1L) a special use permit under § 350-204 is required first: the applicant may go straight to a Planning and Zoning Commission hearing, or the Zoning Administrator 'may administratively approve a special use permit if no written objections are received within 21 days' of posted, published and mailed notice to owners within 300 feet. Written objections in that window push the case to a P&Z public hearing. An approved SUP is recorded with the County Clerk, shown on the Official Zoning Map, and expires if the use has not commenced within, or is discontinued for, one year.

The UDC places no limit on renting an accessory dwelling: there is no minimum lease term, no primary-residence test and no cap on the number of rented units. Short-term rental is governed by a separate chapter of the county code, Chapter 280 (Ord. No. 316-2020, adopted 12-8-2020, amended 3-9-2021 by Ord. No. 320-2021). It defines a short-term rental as property 'rented to a member of the public by a property owner where the renter has not manifested an intent to make the dwelling unit a residence or household,' and § 280-4A requires a business registration certificate from the County Clerk plus a permit from the Community Development Department before operating. The permit costs a one-time $50 (§ 280-5) on top of a $35 annual business registration, and operators must carry at least $1,000,000 in liability insurance, provide on-site off-street parking, post the permit in a street-facing window, cap occupancy at twice the number of beds plus sleeping sofas, and mail notice to owners within 200 feet within 10 days of issuance. Ordinance No. 363-2025 (approved 4-8-2025) added a 5% lodgers' tax remitted monthly or quarterly, and the county began compliance enforcement in 2025. Chapter 280 imposes no zone restriction and no owner-occupancy requirement, so a rented casita is treated like any other short-term rental. The UDC does not authorize selling an accessory dwelling separately from the principal dwelling; it is an accessory use on the same lot, and creating a separate parcel would be a subdivision under Article II.