ADU
Auxiliary Dwelling Unit
An Auxiliary dwelling Unit (ADU) is considered as a separate dwelling on a residential lot that has a separate entry, a parking space, independent access and does not infringe on any setbacks and easements. It can even be sold separately under certain guidelines. An ADU requires a permit as a new construction rather than a remodel/addition type permit. If it were attached to the existing building, it is considered an addition and if it does not have a kitchen, it is not considered a duplex. The ADU should be separated by a minimum of 10 feet from the existing residence.
Since a building permit is required from the City of Austin for Auxiliary Dwelling Units as new construction, it requires a plot plan showing all trees, impervious cover, utilities, setbacks, easements and calculations regarding FAR (Floor Area Ratio) driveways and parking. The FAR is defined to be “livable space” or in general terms as interior living area including the 2nd floor. The total FAR is limited to 40% of the area of the lot. To find the maximum allowable FAR, multiply the area of the lot by .40.
The requirement regarding impervious cover limitations applies to the addition of an ADU which changes the overall totals to the lot. This means that when summa
Auxiliary Dwelling Unit
The minimum lot size for an Auxiliary Dwelling Unit is 5,750 square feet on residential lots zoned SF-3. However, the maximum size of an ADU (Auxiliary Dwelling Unit) on any lot is 1,100 square feet or 0.15 FAR (Floor Area Ratio) whichever is smaller. This means that the total square footage of the enclosed livable living area of the AD cannot exceed 15% of the total lot area.
The Plot Plan must meet all City of Austin requirements regardless of any issues regarding the building design. The production of a plot plan usually requires a survey provided by surveyor that measures and defines critical dimensions and locations of buildings, trees, impervious cover elements and setbacks/easements. These measurements need to be accurate and complete in order to produce an acceptable plot plan required for a permit within the City of Austin. A great deal depends on the number and size of trees on the lot. A separate tree plot plan may be required.
There are several rules that have been enacted that limit coverage over portions of the Critical Root Zones of trees that measure over 19” in diameter. Primarily, one cannot develop or build impervious cover that exceeds 50% of the total CRZ (Critical Root Zone) of all such trees combined on the property including the CRZ of adjoining trees on adjoining lots. Before proceeding, always be sure to verify the size and locations of all trees on the property. If there are several large trees in the area of the new building, a special tree plot plan may be required to show those trees appropriately. It is always recommended that the owner consult with an arborist before proceeding.
In addition, if this Auxiliary Dwelling Unit is located in the so-called “McMansion” area as defined in Austin, the new building must comply with those additional rules which could mean that the new building may need to be more than 10 feet from a property line depending on the roof height. Furthermore, if you construct a second story that floor must be less than 550 square feet. Regardless of McMansion rules, a third story for an ADY is not permissible and the total height must be less than 30 feet regardless.
Even though the City of Austin has removed the requirement for a driveway, the rule still remains that at least one additional parking space be provided for the new unit. The exception to the requirement for a parking space is that if this is not required of the property is located with ¼ mile of an activity corridor served by transit. The map defining these areas is available with the City.
The so-called “Condo Regime” may allow an ADU to be sold separately. This has become more useful to developers who purchase a property, build an ADU and then sell off the front part (the original house) to another party and sell the back part separately upon completion of the new ADU. It only makes good sense to do this if a separate access and area is provided for parking that does not impact the front house. The means, that in certain cases, a buyer could buy the back portion of a property only with access limited to the alley and still be within 10 feet of the front structure.
In any event, all short-term rentals are limited to a maximum of 30 days per year and rental agreements must meet the guidelines of the City. Long term rentals are allowed for an Auxiliary Dwelling Unit.
As one can see by reading all of the different calculations regarding the limitations of the area allowed for an ADU, trying to understand how to go about deciding just how much can be built can be somewhat confusing. All of this must be done even before the decisions is made about the design of the structure itself. Many of these calculations need to be done even before understanding whether or not an ADU is even possible when applying all of the se rules.
The best way to go about this is to do the calculations first by applying all of the rules to determine the total maximum allowable building are for this lot including the tree limitations. Then make a decision about the floor plan and what design can be accommodated in that allowable area. Once this is done, then the Owner can contemplate the actual design of the looks and compatibility of the ADU to the existing residence so that the total package will satisfy the original design ideas of the owner. Also, consideration then needs to be given to whether or not this ADU is to be rented, used for family use or to be sold out as a condo under the “condo regime” rules.
To accomplish all of these factors and to arrive at a final design package it highly recommended that the owner employs the services of an Architect with experience in producing documents suitable for permitting for these types of projects.