ADU rules in Bernalillo County

A detached casita, which the county code calls a "secondary dwelling unit", is a permissive (by-right) use in the A-1 and A-2 Rural Agricultural zones and the R-1 Single-Family Residential zone, and reaches the R-2 and M-H zones because both adopt every R-1 permissive use by cross-reference. Detached is the only form the county recognizes: Section 5 defines a secondary dwelling unit as one "located on the same lot as a primary dwelling unit in a detached building". Ordinance 2025-7, adopted 14 January 2025, created the by-right path; from Ordinance 2022-16 until then a casita always needed a conditional use permit and a public hearing. The by-right criteria are objective: the lot meets the zone's area regulations, both units connect to an approved wastewater system, the casita meets zone setbacks, sits at least ten feet from other structures, stays within the separation and size caps, and no competing second unit already exists on the lot.

Unincorporated Bernalillo County has no attached, internal, garage-conversion or basement ADU. Section 5 requires a secondary dwelling unit to be "in a detached building", and every residential zone permits only "one single-family dwelling or H.U.D. Zone Code II manufactured home per lot" as the principal use. The nearest internal option is a conditional use permit for a "second kitchen within a single-family dwelling", and its own terms forbid it from becoming a separate home: the kitchen must be "incidental to occupancy of the entire dwelling in common by members of one family", and approval "will not result in two separate and distinct dwelling units, each exclusively occupied by only certain family members and separated by a solid wall without a doorway". That provision is the county's mechanism under NMSA 1978 Section 3-21-1(F), which directs zoning authorities to accommodate multigenerational housing by allowing up to two kitchens in a single-family district; it is a shared-household accommodation, not an ADU allowance.

Every zone applies the same cap: the secondary dwelling unit shall "not exceed 1,000 square feet or 50 percent of the size of the primary dwelling unit, whichever is less". On a 1,600 sq ft house that means 800 sq ft, not 1,000. Placement is capped as tightly as size: the casita must sit at least ten feet from any other dwelling unit or structure on the lot and no more than 35 feet from the primary dwelling, stretching to 40 feet on lots of half an acre or more and 50 feet on lots of an acre or more. Height follows the underlying zone, 26 feet or 2 1/2 stories in A-1, A-2 and R-1, and the unit must meet the zone's setbacks and be "designed to be architecturally harmonious with the existing primary dwelling".

Nothing in the secondary dwelling unit provisions requires the owner to live on the property, either at application or on an ongoing basis, and nothing limits occupancy to family members. The six by-right criteria are entirely physical: lot area, wastewater connection, setbacks and separation, size, usable open space, parking, and the absence of a competing second unit. The contrast inside the same code makes the silence deliberate: "accessory living quarters", the older no-kitchen guesthouse use, expressly "shall not be rented or otherwise used as a dwelling unit", while the secondary dwelling unit carries no occupancy or tenure condition at all. New Mexico has no statewide ADU statute that would add or preempt an owner-occupancy rule.

Both the zone criteria and the parking schedule say the same thing: "In addition to the parking required for the primary dwelling unit, there shall be provided a minimum of one space for the secondary dwelling unit." Section 21.A.5 sets the baseline for the house itself at one space per bathroom or fraction of one, and never fewer than two spaces per dwelling unit, so a typical two-bath house plus a casita needs three off-street spaces. There is no transit-proximity, small-unit or corner-lot exemption anywhere in the ordinance.

On a lot that meets the underlying zone's minimum area and the other objective criteria, a casita is a permissive use: no hearing, just county zoning sign-off and a building permit. Bernalillo County runs its own building department, adopting the state-approved codes as the "Uniform Construction Codes of Bernalillo County" for the unincorporated area, so both approvals come from Planning and Development Services rather than the state Construction Industries Division. Where the lot falls short, the same casita is available as a conditional use: an application to the Zoning Administrator, newspaper publication at least 15 days out, mailed notice to owners within 300 feet (the enlarged radius applies to any conditional use "that could result in more than one dwelling unit or mobile home per lot"), on-site sign posting, a public hearing, and terms recorded with the County Clerk that run with the land. Ordinance 2025-7 deliberately struck the minimum lot area condition from the conditional use path so undersized lots would have a route. In the C-N Neighborhood Commercial zone a casita remains conditional-use only and still must meet R-1 minimum lot area.

The zoning ordinance places no rental restriction on a secondary dwelling unit: it may be rented long term, and the criteria never ask who occupies it. That is a pointed contrast with "accessory living quarters", the 500 sq ft no-kitchen guesthouse, which "shall not be rented or otherwise used as a dwelling unit". Unincorporated Bernalillo County has no short-term rental ordinance: Chapter 14 (Businesses) contains no STR or vacation-rental article, Chapter 78 (Taxation) imposes no lodgers' tax, and the zoning ordinance never uses the term, so a casita short-let answers only to state gross receipts tax and any private covenants. A casita cannot be sold separately from the house without first subdividing the lot under Chapter 74, and where the unit was approved as a conditional use the recorded terms bind future owners.