ADU Rules in Collin County: What's Allowed, City by City

There is no single "Collin County ADU rule." Whether you can build an accessory dwelling unit — a casita, garage apartment, backyard cottage, or in-law suite — and how big it can be is decided city by city, and the ADU rules genuinely differ from one town to the next. The honest headline: McKinney is one of the most ADU-friendly cities in the county — it allows them on single-family lots, publishes its own ADU guide, and openly encourages renting one out for income. A neighbor like Plano allows a detached "backyard cottage" too, but only on a lot of at least 6,000 square feet, set behind the main house. Same county, two different rulebooks — which is why the first move is always to confirm the rule for your address, not your neighbor's city.

Below is what actually counts as an ADU, the two Collin County cities whose rules we verified against their own published code, the owner-occupancy and short-term-rental questions that trip people up, and the permit path.

What actually counts as an ADU

An ADU is a complete, independent second home on the same lot as your main house. McKinney's own guide puts it simply: every ADU has a kitchen, a bathroom, living space, and a place to sleep, connected to water and electricity like the primary home. That kitchen is the dividing line. A bonus room over the garage, a finished attic, or a sunroom without cooking facilities is just more house — not an ADU. Add a kitchen and a separate entrance and it becomes a second dwelling, which is what pulls it into the zoning rules below.

You will hear ADUs called a lot of things — casita, granny flat, carriage home, garage apartment, in-law suite — and they take three physical forms: detached (a standalone cottage in the backyard), attached (a wing with its own entrance), or a garage conversion. The word your city uses matters, because some cities allow only kitchen-less "guest quarters" and reserve the term "dwelling" for a stricter, separately regulated thing.

The rules vary by city — here is what is verified

Texas has no statewide ADU law, and Collin County does not set one for its cities. Each municipality writes its own zoning, so the useful thing is not a county rule — it is a concrete sense of how far apart two neighbors can be. Here are two we verified directly against the cities' published materials.

McKinney: among the friendliest (verified)

McKinney treats ADUs as a real housing option and walks homeowners through them in an official guide. If your property is zoned for a single-family home and has room to meet the rules, you can likely build one. The specifics, straight from the city's guide:

• Size is tied to your main home. If the main home is under 1,200 sq ft, the ADU can be up to 75% of its floor area; if the home is 1,200 sq ft or larger, up to 50%. The city's own examples: an 1,100 sq ft home can build an ADU up to 825 sq ft; a 1,600 sq ft home, up to 800 sq ft.
• It cannot be taller than your main house. One-story home, one-story ADU.
• Setbacks match your main home and vary by zoning district.
• Parking: if you already have four or more spaces, you add none; fewer than four, add one; in the Historic Overlay District, three total.
• Utilities are shared. The ADU uses the main home's meters and the same electric service — no separate account or meter.

The catches are real: Planned Development (PD) zones and many HOAs can prohibit ADUs outright, and in the Historic District you need a Certificate of Appropriateness before you build. If your lot sits in Stonebridge Ranch or another master-planned village, check the association's rules the same day you check the city's. Our McKinney additions and ADU page covers how this plays out on real McKinney lots.

Plano: a "backyard cottage," with strings (verified)

Plano allows a detached ADU under its Backyard Cottage program — but gates it differently than McKinney. From the city's fact sheet:

• Minimum lot size: 6,000 sq ft.
• Floor area at least 400 sq ft, and no more than 1,100 sq ft or 50% of the main home — whichever is more restrictive.
• Must sit behind the main house, at least 10 feet from it, on a permanent foundation.
• Must be architecturally compatible with the main home — matching roof profile and pitch, style, and window proportions.
• One additional off-street parking space, and tandem parking does not count.
• It cannot be sold separately from the main house.

Notice the difference: McKinney gates on a percentage of your main home, while Plano gates on lot size, placement, and design compatibility. A plan that sails through in one city can be a non-starter two miles up US-75.

Owner-occupancy, renting, and Airbnb

Three separate questions hide inside "can I rent it out?"

Owner-occupancy. Neither McKinney's nor Plano's published ADU materials spell out an owner-occupancy requirement — but plenty of cities elsewhere do, so confirm yours before you assume you can build purely as an investment.

Long-term renting. This is often the whole point, and it is frequently allowed. McKinney explicitly lists renting one out for extra income as a benefit; Plano lets you rent the cottage or live in the cottage and rent the main house.

Short-term rental (Airbnb/VRBO) is a different rulebook. STRs are governed by a city's separate short-term-rental ordinance and your HOA's covenants — not by the ADU zoning. Being allowed to build and long-term-rent an ADU does not mean you can list it nightly. Confirm the STR rules and your HOA before you bank on nightly income. One more wrinkle: renting an ADU can change the homestead exemption on that slice of your property — we walk through that in will a home addition or ADU raise my property taxes.

Every other Collin County city: confirm before you design

Allen, Frisco, Prosper, Celina, Melissa, Anna, Fairview, Wylie and the rest each set their own rules, and we will not put numbers in your head that we have not verified against a current ordinance. What we can tell you is exactly what to ask your city's planning department:

• Is a detached second dwelling allowed at all, or only kitchen-less "guest quarters"?
• Is there an owner-occupancy requirement?
• Is renting allowed — long-term, short-term, or neither?
• What is the minimum lot size, and where on the lot can it go?
• What are the height, setback, parking, and utility-meter rules?
• And on top of all of it — what does your HOA allow? Architectural review is near-universal in newer Collin County subdivisions and is often stricter than the city.

That last layer is why "confirm with your city" is not a dodge — it is the actual first step. Most cities publish a zoning map and staff a planner you can email before you spend a dollar on drawings.

What an ADU costs (industry ranges, not a TRG quote)

Nationally, 2026 industry data puts a full ADU at roughly $150–$300 per square foot, with HomeGuide's 2026 data citing an average around $180,000 and most projects landing between $80,000 and $250,000 depending on size and finish. A garage conversion is the cheaper path — Angi's 2026 data pegs garage-to-ADU conversions near $110,000 on average, though a detached garage adds 20–30% to run utilities out to it. These are national ranges, not a quote from The Remo Guys — local factors like Blackland-clay foundation work, utility runs, and finish level move the number. For a closer local read, see what a home addition costs in Allen and Plano, or ballpark your own project with our cost estimator.

The permit path

Every ADU needs a permit with a site plan and construction drawings — there is no exception for "it is just a backyard cottage." In McKinney, the whole process runs through the online CSS portal: talk to a planner, pull a personalized checklist from the DNA portal, submit plans, and — if you are in the Historic District — clear the Certificate of Appropriateness first. Then it is the standard inspection sequence, with the trades that are state-licensed in Texas pulling their own permits under yours: electricians and HVAC techs through the TDLR, plumbers through the TSBPE. Other Collin County cities run their own portals, fees, and timelines; our Collin County building permits guide covers how the major cities differ.

Build it as one project, not five

An ADU is a small building with a big coordination problem: a foundation engineered for Blackland clay, a design that reads as part of your home, every trade, the city, and usually an HOA architectural board — all pulling toward one seamless result. That is the argument for a design-build approach, with one general contractor owning the drawings, the permit set, and the build together, so the finished casita looks like it grew there rather than got bolted on. The Remo Guys is an insured and bonded design-build contractor based in Allen, serving McKinney, Plano, Frisco, Prosper, and all of Collin County. Request a consultation, and we will map your ADU's rules, budget, and permit path before you commit.