ADUs in Austin - What you need to know………………………
So you’ve read about granny flats and ADUs and you have a backyard that is calling out to you. Build here, it says. Come join the movement.
The previous generation in Austin felt that a big yard was the ultimate dream. Suburbia was partly built on this philosophy. However, density and walkable neighborhoods with all your basic needs close by have been a staple of great cities for generations. Austin is on the cusp of greatness. The 1200 sq. ft. house on a 9,000 sq. ft. tract of land is a relic of suburbs past.
So what do you need to know?
For starters, you need to know what you want out of this. ADUs can run the gamut from 300 sq. ft. cooled buildings without plumbing to 550 sq. ft. one bedroom apartments above a garage to an 850 sq. ft. two story fully functional small house to 1100 sq. ft. two story houses. The new 1100 sq. ft. plans at 1 bedroom or 2 are very nice, very livable houses. Once you figure out the intended use, here are the procedural items to consider.
Available space to build
You need at least a 5,750 lot. Your combined living area of the existing dwelling and new dwelling can not exceed 40% of the sq. ft. of your lot. So, a 7000 sq. ft. lot would simplistically have a max build area of 2800 sq. ft. for BOTH houses. The max building size for the ADU is 1100 sq. ft. or 15% of the sq. ft. of your lot, whichever is lower. So, the 7000 sq. ft. lot would produce a max ADU of 1050 sq. ft. You would need a 7333 sq. ft. lot to build a max size ADU.
Impervious Cover
Impervious cover is the percent of the lot that is covered with concrete or hard packed gravel (driveways). This includes porches, sidewalks, etc. You are limited to 45% of the lot square footage for impervious cover. This places a premium on two story buildings, driveway strips instead of a full concrete driveway, etc. If you have a 2000 sq. ft. one story house, this is going to make building a full size ADU more difficult with an average lot size. You need more land area than normal with larger one story main houses.
Trees
That large tree that was planted in the middle of your back yard in 1982 that everyone says adds value? It doesn’t. It subtracts value. It’s now in your way. Trees over 19 inches in diameter are protected trees and you can not build within the ½ critical root zone. A tree in the middle of the yard is likely in the worst possible spot. There are remedies, of course, but you need to know where your trees are and the impact and then plan accordingly.
Zoning
In Austin, you need to be SF-3 zoning. SF-2 and SF-3 are the most common types. SF-2 does not currently allow for an ADU.
Flood Plain
Is your property in the flood plain? If so, the ADU can not encroach upon the flood plain. That means you have to build it in the space that is entirely not in the flood plain.
Deed Restrictions
And finally, the bane of Austin’s attempts to become a more dense and greater city, we have deed restrictions. In the 50’s and 60’s and 70’s, apparently building subdivisions with deed restrictions isolating the property to single family were really attractive. Or, developers thought they were. Nonetheless, they persist and last forever, sort of like luggage, unless an intervention is staged by the owners in a subdivision to have the restrictions removed. Make sure your property is not encumbered by deed restrictions. The City of Austin doesn’t care for permitting but your neighbors could care and then you’re left with neighbor on neighbor litigation which is never really a win.