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I want to build an ADU! What do I need to know?

Updated: Dec 30, 2019

Accessory Dwelling Unit (ADU), Granny Flat, Casita, Secondary Dwelling Unit, Secondary Apartment.

We’ve had a lot of interest from clients who are considering adding a secondary unit to their properties. We’ve assembled some quick notes to help you through the process.

I want to build an ADU! What do I need to know?

  1. The good news is the City of Austin has relaxed the rules to make adding an ADU to you property easier!

  • Minimum lot size has been reduced to 5,750 square feet, with SF-3 zoning.

  • Parking regulations have been relaxed if you are within ¼ mile of an area served by public transit.

  • Unit size has increased to either 15% or 1,100 square feet of FAR, whichever is smaller.

  1. Ask yourself - Why do I want to build an ADU?

  • I want a steady rental income to offset my current mortgage.

  • I want to Air B&B and make a killing during special events!

  • Sorry! STR’s (Short Term Rental) are limited to 30 days a year, if you’re operating a legitimate STR.

  • I need a place to move my elderly parents.

  • I’d like a small office outside the confines of my home.

  1. How do I know if it is allowed on my property?

  • Does my zoning allow it?

  • SF-3 zoning with a minimum lots size of 5,750.

  • SF-2 properties allow one residence only.

  • Please let us know if you do not fall within an SF-3 zoning district and we can discuss your options.

  • You can check your property's zoning here.

  1. How much can I build?

  • Technically either 15% or 1,100 square feet of FAR, whichever is smaller.

  • The base building and impervious cover is still 40 and 45% respectively, so-

  • If you have an existing sprawling one story house, or a large amount of concrete drive and walkways on you property the ADU square footage will need to fit within the remaining allowable coverage for building and impervious.

  1. I have trees on my property, close to where I’d like to build- is that a problem?

  • It depends. The City of Austin has very specific tree protections in place, so a few things would need to be determined before knowing if the tree is really a problem-

  • What type of tree is it?

  • How large is the tree?

  • Is the tree healthy?

  1. Do I need to provide parking?

  • It depends.

  • If you live within ¼ mile of an area served by public transit the answer is no.

  1. How much will it cost?

  • It depends. A few factors that can effect the design and construction of an ADU are:

  • How close is the ADU to my existing utilities – water, power, and sewer?

  • Can my existing utilities handle the additional building?

  • I’d like a roof deck to take advantage of my downtown views!

  • Smaller doesn’t mean cheaper.

  • We’ve had people say – “It’s small, so it should be cheaper, no”?

  • Short answer – NO!

  • We can explain this in more detail, but the boilerplate “it costs $X.XX amount per square foot”, simply does not work for smaller projects.

  1. I’ve considered modular construction, what do you think?

  • While modular can work in certain situations, its been our experience that it is not for everybody and:

  • It depends. Why are you considering modular construction?

  • Do you live in an area with a scarce labor supply?

  • If so, then modular construction might be beneficial.

  • Does it have a certain allure for you? If so, please explain.

  • I’ve seen a modular company plan that I like, but I want to make modifications.

  • Modifications are typically not allowed, unless the modification is superficial.

  • I’ve heard it’s much cheaper than site built construction.

  • Modular construction is typically not cheaper.

  • The “it’ll cost $X.XX amount per square foot” you see listed on websites does not factor in:

  • Local permitting

  • Foundation design

  • Shipping

  • Crane set & install

  • Utility connections

If you’d like to discuss your project, we’d love to hear from you.

We can provide a low cost project review in which we will put together a brief feasibility report for your project, based on your initial input. Download your “project worksheet” information here.

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