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Doña Ana County, NM Accessory Structures: ADU Rules (2026)

Significant Restrictions

Compiled from the official code textEditor Martyn O'NeillLast verified August 2026

ADU rules in Doña Ana County, NM, also called accessory dwelling unit regulations or granny flat ordinances, cover setbacks, owner-occupancy, parking, and permit requirements.

Key Facts

D1 ADU size cap: 1,800 sq ft, 1 per lot
D1 ADU permit: Special use permit required
D2 ADU size cap: 800 sq ft
D3 zone: Accessory dwelling not applicable
Rural R5/R5L allowance: 1 per 5 acres, 1,800 sq ft

ADU Eligibility Quick-Check

An Accessory Dwelling Unit (ADU) is a smaller, independent living unit on the same lot as a single-family home: converted garages, detached cottages, basement or attic conversions, and attached additions all qualify. Most states have streamlined ADU permitting since 2020, but local rules still control size, height, and setback.

Is your lot zoned single-family or multi-family residential?
Does your lot have at least 1,200 sq ft of buildable space?
Is your existing home owner-occupied (in most states this is not required, but some require it for ADU permits)?
Will the ADU stay under your state's size cap (commonly 850-1,200 sq ft for detached)?
Can you provide one off-street parking space (waived in many states within 0.5 mi of transit)?

If you answered 'yes' to most of these, you likely qualify in New Mexico under recent ADU streamlining laws. The Doña Ana County, NM rules below cover local size, height, and setback requirements.

Summary

The D1 Low Density Residential zone allows one accessory dwelling unit per lot, capped at 1,800 square feet, but only with an approved special use permit under UDC Table 5.8; the D3 High Density Residential zone does not allow accessory dwellings at all.

These county ordinances apply to unincorporated areas of Doña Ana County. Incorporated cities within the county may have their own rules that take precedence over county-level regulations.

Table 5.8 Site Standards: D1 Zone. Low Density Residential Zone (D1 & D1L) ... 2. Other Lot Standards Accessory dwelling: 1 accessory dwelling limited to 1,800 square feet and permitted by special use permit

View official code

Full Breakdown

Accessory dwelling units, garage apartments and casitas are governed zone-by-zone under § 350-502 rather than as a single countywide rule. " A homeowner cannot simply pull a building permit for a second unit; the request has to go through the § 350-204 special use permit process, reviewed administratively if no written objection is filed within 21 days of notice, or at a Planning and Zoning Commission public hearing if an objection comes in. 4 allows "1 accessory dwelling limited to 1,800 square feet and 1 accessory dwelling permitted for every 5 acres of lot area," with R5 and R5L again requiring a special use permit. An accessory dwelling still counts toward the zone's accessory-building height and setback limits and toward the lot-coverage cap where one applies.

Violations & Fines

Building or occupying an accessory dwelling unit in the D1 zone (or R5/R5L) without first obtaining the required special use permit under § 350-204, exceeding the 1,800-square-foot size cap, or placing a second accessory dwelling on an R5 lot beyond the one-per-5-acres allowance, is a zoning violation; the County can deny occupancy and require removal or permitting correction before the unit may be used.

Frequently Asked Questions

Can I build a guest house or granny flat in the D1 zone?

Yes, but only with a special use permit. Table 5.8 allows "1 accessory dwelling limited to 1,800 square feet and permitted by special use permit" in the D1 Low Density Residential zone, so a standard building permit alone is not enough.

How big can an accessory dwelling unit be?

It depends on the zone. The D1 and rural R5/R5L zones cap it at 1,800 square feet under Tables 5.8 and 5.4, while the D2 Medium Density Residential zone caps it at 800 square feet under Table 5.9.

Are accessory dwellings allowed in the D3 High Density Residential zone?

No. Table 5.10 lists the D3 zone's accessory dwelling standard as "not applicable," meaning second dwelling units are not permitted there under the current UDC.

How many accessory dwellings can I build on a rural R5 lot?

More than one is possible on larger parcels. Table 5.4 allows "1 accessory dwelling permitted for every 5 acres of lot area" in the R5 and R5L zones, each still capped at 1,800 square feet and requiring a special use permit.

Sources & Official References

[1]Doña Ana County UDC § 350-502, Table 5.8

Other rules in Doña Ana County

Noise Ordinances: Quiet HoursSome Restrictions Shed & Carport RulesSome Restrictions Noise Ordinances: Leaf Blower RulesSome Restrictions Noise Ordinances: Barking DogsSome Restrictions Noise Ordinances: Amplified Music & Outdoor SoundSome Restrictions Noise Ordinances: Decibel LimitsSome Restrictions Noise Ordinances: Vehicle NoiseSome Restrictions

All Doña Ana County rules

How Doña Ana County compares: Easiest Cities to Build an ADU·Compare Doña Ana County to another location·View the New Mexico accessory structures overview

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