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This guide is a near-complete resource on how impervious coverage requirements impact the design and development of HOME projects in Austin. At Cedar, we're training models on this criteria to make it easy for you to assess the feasibility of various scenarios (see our previous blog) across any applicable site. A few reasons why we think this is important...

• Impervious cover is one of the biggest barriers to making projects work
• It's often difficult to know what will work on a site until you draw a site plan
• The compact design requirements of HOME require a "Swiss watch" approach to site planning (this is a good thing)
• Cedar does this so you don't have to, training a predictive model to run code compliant options on every site
• While not likely to change in HOME Phase II, modifications to impervious coverage requirements will undoubtedly happen in the future, you'll need the latest data

And many thanks to our friends over at Thrower Design for their review and input on this guide!

Table of contents

1. What is included in impervious coverage?
2. Parking & circulation
3. Driveways & right-of-way
4. Accessibility
5. Front yard development standards
6. Using Cedar to predict/evaluate impervious cover

What is included in impervious coverage?

The most common elements included in the calculation of impervious coverage, and currently supported by Cedar

The City of Austin defines Impervious Coverage as "any type of human-made surface that doesn't absorb rainfall," referenced in Section 25-8-63 of the Austin City Code. The City regulates impervious coverage due to its impact on drainage charge on a site-by-site basis, which impacts water runoff and flooding at the citywide scale.

The recently passed HOME initiative does not significantly impact impervious coverage requirements - 0.45 (or 45%) remains the maximum coverage limit. The ordinance does constrain front-yard coverage to 40%, and "Not more than four parking spaces may be located in the front street yard, or for a corner lot, not more than four parking spaces may be located in the front street yard and side street yard combined."

Parking and circulation

Parking by-unit, individualized access

Cedar's impervious coverage model considers two primary modes of parking on-site — shared rear or front yard parking, and individualized by-unit. Of course you can also select "no parking" and rely on on-street capacity, but we haven't seen this be successful in the Austin market (yet). Our data also shows that parking by-unit and shared rear-yard parking are difficult to make work for (3) units on sites less than approx. 7,500 sf - in these cases, front-yard parking may be the best option.

The most conventional approach to parking is by associating it directly with each unit. This creates increased privacy and convenience for each unit, but can also require increased driveway impervious coverage.

Calculating area by type

	Floor-area-ratio (FAR)	Building Coverage	Impervious coverage
Surface parking
Carport
Garage

Calculating parking areas as required by-connected, including HOME ordinance

Parking by-unit, individualized access

While not common to Austin, rear-yard shared parking is the "urbanist choice" to providing car access while maintaining a strong street presence that maintains neighborhood character. The back-out requirements are significant, and creates shared green space alongside parking, which tends to work better with duplex, triplex, and attached townhome types.

Driveways & right-of-way

Access and back-out

Autoturn simulations (by Cedar + Thrower Design) across scenarios for standard passenger vehicle turning

Conventional two-family projects rarely require significant engineering around vehicular maneuvering and back-out allowances. While technically, three-family projects are considered residential use under the HOME initiative, and likely to align with Type 1 driveway requirements, this does not mean that vehicular access is adequately considered by simply complying. As our favorite advisor and Austin land-planning and urban design legend - Ron Thrower - likes to say...

"Architects and builders must be aware of the spatial needs for vehicles when designing sites with back-out areas to avoid post-construction headaches."

Given the limitations of impervious coverage and additional hard costs, considering driveways wider than (9'-0") to (12'-0") can seem imprudent, but it is necessary in cases of shared driveways and parking. In order to minimize the impacts of vehicular maneuvering and back-out requirements, we've worked with Thrower Design to determine optimized drive layouts across various scenarios.

Driveways & right-of-way

Access by lot type

Cedar's impervious coverage model incorporates access strategies for the most common lot types in Austin, which include Interior Lots, Interior Lots with Alley Access, and Corner Lots (with and without alley access). Residential projects are typically limited to (1) curb cut with exemptions being made for alleys and through variances. While not recommended, it is possible to achieve (2) curb cuts in situations that warrant it.

Right-of-way requirements

Max Driveway Widths

	Max units	Min width (ft)	Max width (ft)
Single-family drive	1	12 ft	20 ft
Two & three-family drive	3	12 ft	20 ft
Multi-family	N/A	20 ft	25 ft

Impervious surfaces within the right-of-way are typically located outside the property line, and therefore are not included in overall area calculations. Driveway widths and sidewalk locations can still impact overall site planning however, and need to be considered. Also, be on the lookout for increased right-of-way widths based on designations that result from changes to roadway types.

Accessibility

ADA and Visitability

Residential projects are not typically subject to ADA but must satisfy Visitability requirements, as dictated by amendment to the Residential Code. The latter focuses primarily on access from “at least one visitable route with a cross slope of no greater than two percent (1:50) that originates from a garage, driveway, public street, or public sidewalk.” Above shows required slopes and access clearances across all potential routes, creating a site plan that is fully in-compliance with accessibility standards.

Using Cedar to predict/evaluate impervious cover

What's included in Cedar's calculations

At Cedar, we're training models on impervious coverage criteria to make it easy for you to assess the feasibility of various scenarios across any applicable site. It's easy to get started, simply login to Cedar, and begin evaluating sites from the map view. For any applicable site, Cedar will tell you which scenarios pass impervious coverage requirements. Soon we'll allow you to customize and optimize by program mix, floor-area-ratio splits, and other assumptions to find ways to achieve compliance.

Did we miss anything?

Let us know by sending a note to contact@cedar.build