Accessory Dwelling Unit

ADUs have been gaining in popularity for years. Over the years I have designed a variety of Secondary Dwelling Units / ADUs / Accessory Dwelling Units, which are different names for all the same elements.

Designing and building an ADU in the City of Austin can be challenging, the rules are constantly changing.** ADUs are typically a lot more expensive to build per square foot, but do offer great flexibilty for homeowners.

Architectural Fees vary for each project but can range from $5,000-10,000.

The designs are influenced by an number of elements;
  1. Trees
  2. Setbacks
  3. Impervious Cover
  4. Building Coverage
  5. FAR

The City of Austin is extremely particular about trees - most any tree other than a few invasives are protected trees if over 19" in diameter or 60" in circumferance.

The rules say that no slab etc can be built within 1/2 of the Critical Root Zone. Driveways and walkways have been permitted in the 1/2 CRZ, yet the COA has gotten tougher on its interpretations.

So the very first step in designing an ADU is to have a licensed survey to both a Tree and Topographic survey, including any trees on adjacent lots that might impact your design.

For the most part setbacks in the City of Austin are standard, however each lot can have unique Easements and/or Deed Restrictions. The COA setbacks are 5' from the side lot lines, 25' from the front and 10' from the rear.

However for a two story ADU, the Mcmansion ordinance tent requirement may shift the ADU away from the property lines, and the height of the ADU is limited to 30' where a principal unit can be 32'.

Impervious Cover - Building Coverage
Impervious Cover is typically limited to 45% although there are some area close to the lake that are limited to 40%. And a few other areas that can have a variety of IC, some 20%, some based on the slope of the lot.

Impervious Cover includes everything that is hardscape, the house, garage, and porches, but also includes walkways and driveways (even if they are gravel). ADUs can be built with and without garages and driveways so this can have a major impact on the overall Impervious Cover.

FAR stand for Floor to Area Ratio. Currently 40% of the lot area (so a 10,000 s.f. lot has 4,000 s.f. of FAR).

ADUs are limited to 15% of the lot area or the maximum of 1,100 s.f. However, there are a lot of factors used to determine FAR which can include detached/attached garages, second floor covered porches, and interior ceilings over 15'. In addition stairs count twice!

FAR is almost always the limiting factor in designing homes and ADUs within the COA.

**This is a brief overvue and should not be consider as advice on the suitability of your property for an ADU. Every effort was made to accurately portray the rules at the time of this writing, however, the rules within the City of Austin are constantly changing. You should hire a qualified Architect and rely on their professional experience to design an ADU specific to you and your lot.**
Homes with Accessory Dwelling Units