Home Addition Design in Texas: What It Really Takes, and What It Costs

Four different projects get called the same thing, and they are not equally difficult.

• Attached addition. A room, bump-out, or second story built onto your existing house. It ties into your foundation and roof structure.
• Detached ADU. A separate small home on your lot. Also called a granny flat, casita, or backyard cottage.
• Garage conversion. Turning vehicle space into a studio or apartment.
• Interior conversion. Finishing an attic or converting existing rooms into a self-contained unit.

Three of these alter your existing structure. One does not. That difference drives everything below.

Because you are operating on a house that is already standing.

An attached addition means cutting into a load-bearing wall, tying new framing into an existing roof, and connecting to a foundation that was never designed to carry it. Every one of those requires professional judgment about a structure nobody fully documented when it was built.

That is architect and structural engineer territory. Not because of red tape, but because getting it wrong damages the house you already own.

A detached structure carries none of that risk. It stands on its own footings and touches nothing.

For a lot of Texas homeowners, yes.

An ADU gives you a guest suite, a home office, or a rental without touching the house. In Austin and Houston, a well-built ADU can generate $800 to $1,500 per month in rental income.

It is also the version with the cleanest permitting path, since you are not asking a reviewer to evaluate how new work attaches to old work.

The tradeoff is land. You need the lot space and the setback room, which is exactly what your site plan has to prove.

Texas sets no statewide ADU rules, so this changes completely depending on where you live.

• Houston. ADUs up to 900 square feet, attached or detached, as long as the unit has a full kitchen and bathroom and meets building codes and setback rules. Houston has no zoning code, which makes ADUs broadly permissible.
• Austin. Up to 1,100 square feet or 0.15 floor area ratio, whichever is smaller, with at least 10 feet of separation from the primary home. The 2023 HOME Ordinance allows up to three residential units on most single-family lots, and there is no owner-occupancy requirement.
• Dallas. Harder. ADUs are not permitted as-of-right in most single-family zones. You need an Accessory Dwelling Unit Overlay or a board of adjustment exception.
• San Antonio. Generally requires the owner to live on site, in either the main house or the ADU.

Check your city before you fall in love with a floor plan.

More than the construction quote you were given.

ADU construction in Texas typically runs $100,000 to $180,000 depending on size and finish level. That figure usually excludes the parts that surprise people.

Site prep can add $30,000 to $50,000 on a lot with slope, rock, or mature trees. Hill Country lots are notorious for this.

You will also need a current survey showing lot lines, easements, and existing structures. Budget $500 to $1,000 and get it early, because the survey from when you bought the house is often not accurate enough for permitting.

Then add city permit fees and a review period that generally runs 30 to 60 days once your application is complete.

This is the most expensive mistake in Texas ADU projects.

Private deed restrictions and HOA rules operate independently of city zoning. They can prohibit detached structures, cap square footage, or ban rental use entirely, and a city permit does not override them.

People get plans drawn and contractors hired before discovering their HOA prohibits the structure outright.

Read your deed restrictions first. It costs you an afternoon instead of thousands.

We want to be direct about scope, because the wrong tool on this project is expensive.

NALO handles detached ADUs within the size limits, along with patio covers, decks, pergolas, and carports. These fall inside the Texas Occupations Code Section 1001.056 exemption, which generally covers residential structures under 5,000 square feet with no clear span over 24 feet.

Houston's 900 square foot cap and Austin's 1,100 square foot cap both sit comfortably inside that.

NALO does not handle attached additions, garage conversions, or interior conversions. Those alter existing structure and need a licensed professional.

For a detached ADU, you get permit-ready plans in about 30 minutes to an hour for $119, validated against your city's adopted code before you download. A photorealistic 3D render of the finished structure is $229, or $299 bundled.

Hire a licensed professional if your project involves:

• Cutting into or removing a load-bearing wall
• Tying new framing into your existing roof or foundation
• A second story on the primary house
• Any structure spanning more than 24 feet between supports
• A slab on expansive soil, or windstorm certification in a coastal county

We would rather send you to the right professional than sell you plans your city will reject.

• Enter your address. Your city's code loads immediately, including setbacks, height limits, and the required sheet list.
• Answer plain questions. Dimensions, materials, structure. No technical vocabulary.
• Upload photos and your survey. These place the ADU accurately against your real lot lines.
• Download your plans. A 10-sheet PDF drawn to NCS and AIA standards at tabloid size, ready to submit.

If an input breaks a rule, such as insufficient separation from the main house, you find out before the plan is generated rather than at the counter.