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Calculating impervious cover limits for mixed use projects located in suburban watersheds within the City of Austin could make your head spin. Our Austin team breaks down the regulations in simple terms.
The total allowable impervious cover in the City of Austin is generally determined by examining two competing regulations: the zoning ordinance and the watershed ordinance, where the more restrictive will govern. For the most part, watershed regulations will be more restrictive than zoning regulations. In Suburban Watersheds, projects are broken out into three project categories: single-family residential, multifamily residential, and commercial, with the following impervious cover regulations:| Land Use | Maximum Impervious Cover |
| Single Family Residential | 50% (lots >5,750 sf) 55% (lots <5,750 sf) |
| Multifamily Residential | 60% |
| Commercial | 80% |
It seems simple enough until we consider a scenario where a mixed-use building is proposed with both commercial and multifamily residential components. Austin’s Land Development Code does not offer guidance as to how to calculate our impervious cover limit, so we have to turn to the Environmental Criteria Manual (ECM) for the answer. Appendix Q-5 of the ECM provides a “simple ratio” formula to calculate impervious cover (IC) for mixed-use projects, shown below. Multifamily residential and commercial impervious cover limits are 60% and 80%, respectively.
Total IC = A*60% + B*80%
A = % of multifamily ground floor
B = % of commercial ground floor use
60% = Multifamily IC allowance
80% = Commercial IC allowance
If, for example, we have a 1-acre site where 75% of the building ground floor area is dedicated to multifamily uses and 25% of the building ground floor area is dedicated to commercial uses, the impervious cover limit will be calculated by multiplying 75% of the gross site area (0.75 acres) by 60% (the multifamily use impervious cover limit) and multiplying 25% of the gross site area (0.25 acres) by 80% (the commercial use impervious cover limit) and adding the resulting figures together:
Total IC = A*60% + B*80%
A = 0.75 ac (% multifamily ground floor use)
B = 0.25 ac (% commercial ground floor use)
Total IC = 0.75*60% + 0.25*80%
Total IC = 0.2ac + 0.45ac
Total IC = 0.65 acres
This formula allows the developer to take into account the different impervious cover limits for commercial and multifamily uses per Suburban Watershed regulations, instead of making the assumption that the more restrictive limit will apply.