Things to Consider when Looking for Land to Build On: Part II – McMansion , FAR and Impervious Coverages

This is the second installment in our blog posts about things to consider when purchasing a residential lot for construction. The first installment discussed City of Austin Setback Requirements. This installment will look at the City’s Sub-Chapter F ordinance known as the “McMansion Ordinance” as well as looking closer at how the city defines FAR and Impervious Coverages and how they are calculated. Note that all these calculations and building setback planes must be submitted to the city at the time of permitting, a licensed architect will take all of this into consideration when designing and should set up code review sheets to lay out the information for the city.

1. The McMansion ordinance came about in 2006 as a reaction to small central Austin bungalows being remodeled into huge property filling houses.  There is still much debate around it which constitutes its own series of blog posts but in an effort to avoid divergence from our topic at hand we will just accept it as a fact and explain it some. The ordinance effects the following building areas on a site:

The heavy blue line indicates the “tent” formed by the side and rear setback planes. The buildable area is the smallest area included within the front, side, and rear yard setbacks; maximum height limit; and the combined side and rear setback planes (shown here as the green area).

  • Building Height limiting it to 32′
  • Building Setbacks The COA defines “setback planes”, for the side yard and back yard setbacks.  These are found by taking the 5′ side yard setback, extending it up into space 15′ and then angle it 45deg towards the center of your lot.  The 15′ is measured from the ground at 40′ intervals to take into account sloping site terrains.   There are of course different exceptions allowing for dormers and gables.  This is best explained with an image – most architects I know have to look it up for every new project because the ordinance just does not make sense.  Keep in mind these setbacks are in addition to the setbacks discussed in our previous post.
  • Side Wall Articulation (Existing Side Wall Exceeds 32 Feet) Articulation is required for side walls on additions or new construction that are 15 feet or taller and located within 15 feet of the side lot line. No wall may extend for more than 32 feet without a projection or recession of at least 4 feet in depth and 10 feet in length.

    Sidewall Articulations:  The code requires that your building jog if a sidewall is too long.  “A side wall of a building that is more than 15 feet high and is an average distance of 15‘ or less from an interior lot line may not extend in an unbroken plane for more than 32’ along a side lot line.”

2. FAR – Floor to Area Ratio: This is the amount of building the city will let you have on a lot after all other setback and impervious restrictions are met.   “FAR measures the square footage of a building relative to the amount of land on which it sits . The City of Austin defines FAR as the ratio of the gross floor area (total square footage of building on all floors with some exceptions naturally) to the gross site area (total square footage of lot). For example, if a home is 4,000 square feet in size and the size of the lot is 10,000 square feet, the FAR is 4,000 divided by 10,000, which is equal to 0.40.”  Most residential zoning in Austin has an FAR of .4, this is determined by your lots zoning and any affecting overlays in the area. The city has an online calculator you can use here.

3. Impervious Coverage– Increased impervious coverage in the city results in increased runoff which has negative affects on our creeks, lakes and water reservoirs.  For most residential lots in the City of Austin the Impervious Coverage allowance is 45%.  This means that 45% of your property overall area can be covered by surfaces that do not allow water to penetrate the ground.  Now with all things city related there are exceptions.  The city will calculate uncovered decks at 50% for instance and water features (including water harvesting equipment) are not counted.

Natural and Impervious Cover Diagram from the EPA

Much of the above discussion centers around things that will dictate what you build on a lot but it is good to know up front prior to purchasing a lot in the City of Austin.  While there are numerous benefits to living in the City there are likewise numerous restrictions to building here!

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About clarkrichardson

Clark Richardson Architects practice design with the goal of creating contemporary architectural experiences through form, sustainability and detail. We specialize in green building, sustainable residential and commercial design and have architectural studios in Santa Fe and Austin and a design studio New Orleans.
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