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mad works architecture +, pc

Austin, TX 78702

(512) 829-7295 



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

January 24, 2018

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.

  2. 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.

  3. 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.

  4. 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.

  5. 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?

  6. 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.

  7. 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.

  8. 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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June 28, 2018

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