ADU Cost in North Texas (2026): Garage Conversion vs. Detached Build

An ADU — an accessory dwelling unit, also called a casita, granny flat, or garage apartment — is a complete, independent second home on your lot: its own kitchen, its own bath, its own entrance. In 2026 that independence costs roughly $150 to $300 per square foot to build (Angi and HomeGuide 2026 data), and total budgets run from about $60,000 for a simple garage conversion to $400,000 for a detached new build — a typical market range, not a quote from The Remo Guys. Which of the three common paths you take — convert a garage, build above one, or put up a detached unit — moves the number far more than any tile or countertop you'll choose. And in Collin County, one question comes before all of that: whether your city will even let you build a rentable ADU.

Can you build an ADU here at all? The Collin County zoning gate

Start here, because a national cost calculator won't. Texas has no statewide law forcing cities to allow ADUs. A 2025 bill that would have made ADUs legal "by right" in single-family zones statewide — Senate Bill 673 — stalled and died in the legislative session: its last recorded action was being placed on the House General State Calendar on May 26, 2025, and it never reached a floor vote before the session ended. That leaves every city writing its own rules, and North Texas suburbs tend to write them tight.

McKinney, for example, permits an accessory dwelling or guest quarters that's tied in size and height to your main house — but its zoning treats that space as incidental to the main home, so renting it out as a separate household generally requires a Specific Use Permit (SUP), reviewed by Planning & Zoning and City Council, not a by-right approval. Allen, Plano, and Frisco each set their own zoning approval, size caps, and owner-occupancy conditions, and a detached or converted ADU still needs full building, electrical, plumbing, and mechanical permits. The practical upshot: confirm your city allows the ADU you want, at the size and use you want, before you spend a dollar on design. Our Collin County building-permits guide walks the process city by city, and the planning counter is a free phone call.

What an ADU costs by type in 2026

The three build paths carry genuinely different economics. Here's how 2026 national data frames them — directional market ranges, not a bid on your lot:

ADU path	2026 cost (per finished sq. ft.)	Typical total	What you're mainly paying for
Garage conversion	~$150–$400	~$60,000–$150,000	reusing the slab, walls, and roof; new kitchen, bath, MEP, egress, insulation
Above-garage apartment	~$200–$350	~$80,000–$300,000	reinforcing the garage, a new upper floor, a staircase, a full unit
Detached new build	~$200–$400	~$150,000–$400,000	a whole new engineered foundation, structure, and long utility runs

Sources: garage-conversion figures track Angi's 2026 garage-to-ADU data (average near $110,000, range $60,000–$150,000); above-garage figures track Angi's 2026 garage-with-apartment data ($80,000–$300,000); detached figures track HomeGuide's 2026 ADU data ($150,000–$400,000). Texas projects tend to sit below high-cost coastal markets — detached ADUs in California commonly run $375 to $600+ per square foot — but Collin County's engineered foundations and finish expectations push toward the upper half of the national band.

One number will look strange: a small garage conversion can post a higher price per square foot than a big detached build. That's not a mistake — it's the tell for how ADUs actually price.

Why the per-square-foot number understates a small ADU

An ADU is a tiny house, and a tiny house is mostly fixed costs that don't shrink with size. A 400-square-foot unit and an 800-square-foot unit each need one full kitchen, one full bathroom, one entrance, one heating-and-cooling system, and one set of utility connections. Spread those over fewer square feet and the per-foot rate climbs. Four drivers do most of the work:

A full kitchen — not just a room. This is the line that separates an ADU from a bedroom addition. Kitchen cabinets alone run roughly $2,000 to $13,000 installed for a full set in 2026 (Angi), and Angi pegs the average kitchenette add — cabinets, counters, appliances, and the electrical and plumbing to serve them — at about $15,750. So even a compact ADU kitchen rarely lands under five figures. A room addition or a primary suite can skip this entirely; an ADU cannot.

A full bathroom. A bath built into new construction runs about $400 per square foot in 2026 (HomeGuide) — several times the cost of the living space around it — because it concentrates plumbing, waterproofing, tile, and fixtures into a few feet. How far the new bath and kitchen sit from your existing sewer and water lines is the hidden lever on the whole job.

Utility and MEP tie-in. Every ADU needs water, sewer, electrical, and usually gas. Tying a conversion into a stub-out a few feet away is cheap; trenching new lines 40 to 80 feet to a detached unit at the back of the lot runs roughly $6,000 to $18,000 (2026 ADU utility data), and full utility connections across all trades commonly total $5,000 to $30,000. If your city requires a separate meter, tap and impact fees stack on top. And in Texas the wiring and plumbing aren't a DIY option: electrical and HVAC work must be done by TDLR-licensed trades and plumbing by a TSBPE-licensed plumber — that licensing is real and worth insisting on.

The foundation — on Collin County clay. A detached ADU needs its own engineered foundation, and most of Allen, Plano, Frisco, and McKinney sits on expansive Blackland Prairie clay that swells and shrinks with moisture. That usually means an engineered post-tension slab or drilled piers designed off a geotechnical soil report — not a commodity slab. A garage conversion sidesteps a new pour, but inherits the existing slab: garage floors are often thinner, uninsulated, and never poured to living-space standards, and if the clay has already moved one, remediation can eat into the savings.

Garage conversion, above-garage, or detached: the honest trade-offs

A garage conversion is the cheapest ADU because the expensive shell — foundation, walls, roof — is already standing, which is why Angi's 2026 data centers a garage-to-ADU conversion near $110,000, well below HomeGuide's $150,000-to-$400,000 range for a detached build. If you have a garage you don't need for cars, it's often the smart, honest choice. But be clear-eyed: you lose covered parking (some HOAs and cities won't let you give it up), the space has to be brought up to habitable code — egress windows, insulation, ceiling height, moisture control, ventilation, none of it visible in a per-foot headline — and a converted garage rarely feels or appraises like a purpose-built cottage.

Building above the garage keeps your parking and adds the most flexible space, but you pay to reinforce the garage to carry a second floor and to add a staircase, which is why above-garage units price closer to detached construction than to a simple conversion.

A detached unit is the most expensive path and the most disruptive to the lot, but it's usually the best long-term asset: a real cottage that reads as a true second dwelling to appraisers and future buyers. For a pure rental or a nearby-family suite, conversion is frequently the right call; for flexibility and resale, detached often earns its premium.

How to budget before you have a bid

You don't need a firm number to plan well — you need the right sequence:

1. Confirm zoning first. Size cap, use (rental vs. family), owner-occupancy, parking. Everything downstream depends on it.
2. Pick your path honestly. Conversion, above-garage, and detached are three different budgets and three different appraisals.
3. Price the fixed costs up front. Kitchen, bath, and utility connections cost about the same whether the unit is 400 or 800 square feet — get real allowances on them early.
4. Scope the soil and foundation work for any detached build; on clay it's the spine of the estimate, not an add-on. Budget design and engineering too — 2026 industry guides put a full ADU plan set at roughly $5,000 to $30,000, and that happens before a shovel moves.
5. Budget the carrying cost. New finished square footage raises your appraised value, so plan for a higher tax bill — our guide on whether an addition raises your property taxes shows the math.

For a directional starting point before a site visit, our cost estimator turns your scope into a ballpark range.

Build it with the whole picture in view

An ADU touches zoning, soil, structure, a full kitchen and bath, utilities, and permits all at once — which is exactly why a single-trade quote and a real number rarely match. The Remo Guys is an insured and bonded design-build general contractor based in Allen, building home additions and ADUs across Collin County — Plano, Frisco, McKinney, and beyond — since 2019. We'll check your city's rules, order the soil work, coordinate the licensed trades, and give you a real, itemized number for your unit. Request a consultation and let's map it out.