ADU rules in Lewisville

Lewisville's residential ADU is called a backyard cottage: a detached, self-contained dwelling unit accessory to the main house (an accessory building with bath and kitchen facilities is automatically treated as one). One cottage is allowed per lot, behind the principal building. It is allowed by right in the AO, R-18, R-12, R-9, and R-7.5 single-family districts and in the OTMU-1 and OTMU-2 Old Town Mixed-Use districts, and by Special Use Permit in R-6, R-5, and ETH.

The UDC's only residential accessory dwelling use is the backyard cottage, which is defined as detached; no attached ADU, garage apartment, or interior secondary-unit use appears anywhere in the use tables, and the UDC states that unlisted uses are not allowed. The separately listed Accessory Dwelling Unit (Commercial) is a caretaker unit inside a nonresidential building, not a homeowner ADU. This is a conclusion from the use scheme rather than an express prohibition sentence: the code offers no attached-ADU path rather than a written ban.

The size formula favors larger homes: because the cap is 800 sq ft or 50 percent of the principal building whichever is greater, a 2,400 sq ft house could support a 1,200 sq ft cottage, while every cottage must be at least 400 sq ft. Height is capped at the height of the main house rather than a fixed number of feet. The cottage also needs 10 ft of separation from the house.

The property owner must occupy either the principal building or the backyard cottage as a permanent residence, so an investor cannot rent out both units. The requirement is written into the cottage standards themselves and applies in every district where cottages are allowed. The cottage also cannot be sold separately from the main house.

The parking-by-use table lists Backyard Cottage at 1 space per dwelling unit in both residential and mixed-use districts. The cottage standards add the placement rules: a space outside a garage cannot be stacked behind another car and has to adjoin the existing driveway.

In the residential use table, Backyard Cottage is marked A (allowed) in AO, R-18, R-12, R-9, and R-7.5 and S (Special Use Permit) in R-6, R-5, and ETH; in the mixed-use table it is A in OTMU-1 and OTMU-2 and not allowed in OTC, MU, and MU-90. On most standard single-family lots a cottage is by right with a building permit only, but the small-lot R-6 and R-5 districts and ETH require a discretionary Special Use Permit. Cottages are not allowed at all in duplex, townhouse, multifamily, manufactured-home, and nonresidential districts.

Lewisville regulates short-term rentals through a permit scheme in the Code of Ordinances (Ch. 4, Art. VI, Div. 4) plus spacing and density limits in UDC §VII.4.3; neither provision specifically addresses backyard cottages, so whether a cottage can itself hold an STR permit is not spelled out in the text read. The prior interim STR prohibition (former §VII.2.1.5) was repealed by Ord. 0800-25-ORD in October 2025.