Building on a slope in Austin: gradient limits, cut and fill

The gradient limits, where they apply

Outside urban watersheds, §25-8-302(A) sets two hard prohibitions:

• no building or parking structure on a slope with a gradient of more than 25 percent; and
• no parking area — a surface lot, as opposed to a structure — on a slope of more than 15 percent.

Between 15 and 25 percent a building is permitted, but §25-8-302(B) attaches conditions that shape the design:

• impervious cover on slopes over 15 percent may not exceed 10 percent of the total area of those slopes;
• the terracing techniques in the Environmental Criteria Manual are required for construction uphill or downhill of the slope; and
• hillside vegetation may not be disturbed except as necessary for construction, and disturbed areas must be restored with native and adapted vegetation.

That 10 percent figure is the one that decides schemes. It is not 10 percent of the lot — it is 10 percent of the area of the steep slopes themselves, which on a genuinely hilly site is a small number.

Getting to the site: the driveway rule

§25-8-301 handles access separately, and it is stricter than the building rule. No roadway or driveway may be built on a slope of more than 15 percent — unless the construction is necessary to provide primary access to either:

• at least two contiguous acres with a gradient of 15 percent or less; or
• building sites for at least five residential units.

So a single house on a steep site can be caught in an awkward position: the building might be permissible at 20 percent, while the driveway needed to reach it is not, because one house is neither two acres nor five units.

Note this is a different rule from the 14 percent driveway grade cap in Chapter 25-6, which applies in water supply watersheds and the Barton Springs Zone. Two separate provisions, two separate thresholds, and both can apply to the same driveway.

Four feet of cut, four feet of fill

§§25-8-341 and 25-8-342 both start the same way: cut may not exceed four feet of depth, and neither may fill. Then both list exceptions, and the first is in an urban watershed.

The exceptions that matter on an ordinary residential project:

• Urban watershed — the caps do not apply.
• Building foundation or swimming pool — excepted from the cut limit outright.
• Foundations with sides perpendicular to the ground, or pier and beam construction — excepted from the fill limit.
• Utility construction or a wastewater drain field — excepted from both, with cut requiring the area to be restored to natural grade.
• Roadway right-of-way — excepted from both; rail right-of-way for fill.

Note the asymmetry in the foundation exceptions. A swimming pool is named in the cut list and not in the fill list — which makes sense, since a pool is an excavation, but it is worth reading the right section rather than assuming symmetry.

Further exceptions exist for state-permitted landfills, sand and gravel excavation in the ETJ, and multi-use trails. Those carry their own conditions and are not summarised here.

The detention basin exception has three strings attached

Cut or fill for a water quality control or detention facility — including swales, drainage ditches and diversion berms — may exceed four feet, but only where all three of these hold:

• the design and location of the facility within the site minimize the amount over four feet;
• the excess is the minimum necessary for the facility to function properly; and
• it is not on a slope over 15 percent and not within 100 feet of a classified waterway.

That last condition is the binding one. A detention basin naturally wants to sit at the low point of a site, which is often exactly where the creek buffer is — and the critical water quality zone may already prohibit it there anyway.

Steep lots get flagged on the plat

§25-8-303 requires the constraint to travel with the land. A preliminary subdivision plan proposing a single-family lot on a slope over 15 percent must carry a plan note identifying the lot, and the final plat must carry a plat note identifying it and stating the impervious cover and construction requirements.

Useful when buying: if a lot is subject to the slope rules, the plat should say so. Its absence is not proof of anything, but its presence is a clear signal about what the site will and will not take.