Very generally speaking, the biggest you can build in Texas without a permit is 200 square feet.
Building a Structure Without a Permit For instance, the city of Houston exempts permit requirements for residential accessory structures, including storage sheds, as long as they are less than 200 square feet. However, if your city requires a permit for the shed, then you could face penalties.
A building permit is required for most residential projects inside the corporate city of Houston limits; these include, but are not limited to: Construction work on new or existing buildings.
Non-Residential Buildings - All non-residential buildings require a building permit. Residential Buildings - One & two-family dwellings and townhouses one-story detached accessory structures that are 200 square feet or less used as tools and storage sheds, playhouses, and similar uses do not require a building permit.
Permits are required when building, remodeling or enlarging a building. Permits are also required for other improvements to your property. Projects that are cosmetic, such as painting, wallpapering, carpeting, cabinets and trim work, etc., do not require permits.
Residential permits apply to any development that occurs on private property and not for commercial use. Examples would include: mobile homes; additions to existing homes; swimming pools; porches; private paving; garages; new home construction; patios; and, any structural repair.
In San Antonio, daytime is defined as 6 AM to 10 PM but in Houston permitted construction hours are 7 AM to 8 PM while Dallas ends the daytime hours at 7 PM. Additionally, each jurisdiction regulates weekend, holiday and after hours work differently, requiring special permits for exceptions to normal permitted hours.
The statute of repose for construction defects is ten years from the date of substantial completion of the construction project. This means that even if you discover a defect late, you cannot file a lawsuit more than ten years after the project is finished.
TL;DR: Most buildings in Texas need a permit; exceptions include sheds <200 sqft, decks <30 inches, and fences <7 feet. Permit is required for plumbing, electrical work, residential additions, pools, and concrete pouring.
While selling a house with unpermitted work is not ideal, it is possible. Most jurisdictions require sellers to disclose known issues with the property to potential buyers, including any unpermitted work. Failure to disclose such information can lead to legal action from the buyer, including lawsuits for nondisclosure.