Thoughtful ADU and Casita Design in Doña Ana County

Thinking about adding a backyard casita or converting space into an ADU in Doña Ana County? You’re not alone. Whether you want room for family, a private guest space, or rental income, the right plan can add comfort and value. In this guide, you’ll learn the local rules, permits, smart design for our desert climate, and what to expect with taxes and rentals. Let’s dive in.

ADU basics and zoning

Your first step is simple: confirm where your property sits. Rules differ inside Las Cruces city limits and in unincorporated Doña Ana County.

• Inside Las Cruces: ADUs are allowed “by right” in residential zones, subject to standard dimensional and building code rules. See the city’s overview of land‑use and development to confirm details and process. Las Cruces land‑use and development
• In unincorporated Doña Ana County: The Unified Development Code generally allows one accessory dwelling per lot in many zones, with size, setback, and yard coverage limits that vary by zone. Typical neighborhood setbacks run about 3 to 5 feet on sides and rear, and some zones cap accessory dwellings up to about 1,800 square feet. Check your parcel’s exact zoning table before you design. Doña Ana County UDC

If your property is in a rural zone, you may face acreage limits or need a conditional or special use permit. The code also treats accessory buildings without kitchens differently from full accessory dwellings, so plan your layout accordingly. Doña Ana County UDC

Permits and timelines

Expect a building permit and a zoning/site review before construction. Doña Ana County Building Services posts forms, fee schedules, and submittal requirements online. If your project is inside the city, work with the City of Las Cruces permitting team. Doña Ana County permits

A typical path looks like this:

• Pre‑design check: 1 to 2 weeks to confirm zoning, size, and setbacks.
• Design and plans: 2 to 8 weeks depending on complexity and team availability.
• Permit review: 2 to 8+ weeks depending on completeness and whether you need variances or special approvals.
• Inspections and final sign‑offs: Do not occupy the ADU until you have final inspections and a Certificate of Occupancy. Doña Ana County permits

Wastewater and utilities

Sewer vs. septic often decides cost and schedule.

• If municipal sewer is available, a tie‑in is typically required. Coordinate new water, sewer, and gas service early to confirm capacity, fees, and lead times. Las Cruces Utilities customer central
• If you need on‑site wastewater, the New Mexico Environment Department requires a liquid waste (septic) permit before you build. Your application includes a site diagram, floor plan with bedroom count, soils information, and more. Conventional systems may review in about 5 to 10 business days, with longer times for advanced systems. Final inspection is required before covering the system. NMED liquid waste permitting process

Electrical service in the region is commonly provided by El Paso Electric. Contact the utility early for new meter and service coordination, especially if you plan separate metering.

Site and design tips

Design for our cool‑desert climate and the latest codes.

• Energy code: New Mexico adopted the 2021 energy codes, effective for permits issued on or after July 30, 2024. Plan for higher insulation values, efficient HVAC, and EV‑ready capacity where required. These features may raise upfront cost but lower ongoing bills. 2021 energy code adoption
• Climate‑smart design: Use shading, overhangs or porches, and high‑performance glazing to reduce summer cooling loads. Pair insulation with smart orientation and water‑wise landscaping to cut outdoor water use.
• Accessibility: If you plan for aging in place or multi‑generational living, consider a zero‑step entry, wider doors, and an accessible bathroom. It is easier to include these during initial construction.
• Prefab/modular options: Some modular casita products hold New Mexico approvals, which can speed builds. You still need local installation approvals and inspections. Verify both state modular approval and local acceptance. State modular approval example

Costs can vary widely. Infill lots with existing utilities are often more economical than rural sites that need long utility runs, a well, or a new septic system. Get multiple bids and verify contractor licensing.

Financing, taxes, rentals

• Financing: Owners often use cash, construction loans, home equity loans or HELOCs, or renovation mortgage products that lend against the after‑improved value, such as FHA 203(k) or Fannie Mae HomeStyle. Ask local lenders which programs fit your project and timeline.
• Property tax: New ADUs are typically added as new improvements and increase assessed value. For specifics, contact the Doña Ana County Assessor’s Office. Assessor’s Office
• Short‑term rental use: If you plan to use the ADU as a vacation rental, Doña Ana County requires registration, compliance with its STR ordinance, and collection and remittance of lodgers’ tax. Active enforcement began in mid‑2025. Rules can differ inside Las Cruces city limits, so confirm both jurisdictions before you market the unit. County STR enforcement update

Step‑by‑step checklist

1. Confirm jurisdiction and zoning for your parcel. Rules differ between Las Cruces and unincorporated county. Doña Ana County UDC
2. Call the permitting office for a zoning pre‑check and accessory dwelling guidance. Doña Ana County permits
3. Determine sewer availability or start the septic permit process with NMED if needed. NMED liquid waste permitting process
4. Coordinate utilities early to confirm capacity, fees, and lead times. Las Cruces Utilities customer central
5. Gather contractor bids and choose financing that fits your budget and schedule.
6. Submit building and wastewater permits, schedule inspections, and obtain your Certificate of Occupancy and Permit to Operate before move‑in. Doña Ana County permits

Ready to plan?

A well‑designed casita can expand your lifestyle today and support your long‑term goals tomorrow. If you want help selecting the right lot, coordinating a remote build, or navigating local rules with confidence, reach out to Billie Haynie for patient, end‑to‑end guidance rooted in southern New Mexico experience.

FAQs

Can you build an ADU on your lot in Doña Ana County?

• It depends on your parcel’s zoning and jurisdiction; ADUs are allowed by right in Las Cruces residential zones, while county rules vary by zone and sometimes require special approvals. Las Cruces land‑use and development

Do detached casitas need a septic permit in areas without sewer?

• Yes, if no municipal sewer is available, NMED requires a liquid waste permit before construction, plus final inspection and a Permit to Operate. NMED liquid waste permitting process

Will a new ADU raise property taxes in Doña Ana County?

• Most likely, since it is new construction added to your assessed value; contact the Assessor’s Office for specifics on how your property might be affected. Assessor’s Office

Can you use a casita as a short‑term rental in Doña Ana County?

• Possibly, but you must follow the county’s STR ordinance and lodgers’ tax rules, and confirm city requirements if inside Las Cruces. County STR enforcement update

Are modular ADUs allowed in New Mexico?

• Some modular ADU products hold state approvals; local officials still review installation and plans, so verify both state approval and local acceptance. State modular approval example