Austin floodplain rules: what the 100-year floodplain actually means here

Austin's 100-year floodplain is not always FEMA's

This is the part that catches people, and it is worth reading twice. §25-7-6 defines the regulatory 100-year floodplain in three limbs:

• Areas amended for Atlas 14 — the 100-year floodplain calculated under fully developed conditions, per the Drainage Criteria Manual as amended for Atlas 14.
• Areas not yet amended for Atlas 14 — the 500-year floodplain, either as shown on the FEMA Flood Insurance Rate Map as of 6 January 2016 (as subsequently revised), or as calculated under existing conditions using pre-Atlas-14 data.
• The Colorado River — the 100-year floodplain on the FEMA map dated 6 January 2016, as revised.

Read the middle one again. In areas Austin has not yet updated, the regulatory "100-year floodplain" is the FEMA 500-year floodplain. A lot can sit comfortably outside the FEMA 100-year line and still be inside Austin's regulatory 100-year floodplain for every purpose in this chapter.

"My flood insurance says I'm not in a flood zone" and "I can build here" are answers to two different questions.

The rule is refusal, not a design constraint

§25-7-92 does not regulate how you build in a floodplain. It stops the application being approved:

• (A) — a development application may not be approved if a proposed building or parking area encroaches on the 25-year floodplain, except as §25-7-96 provides.
• (B) — nor if it encroaches on the 100-year floodplain, except under §§25-7-93, 25-7-94 and 25-7-95.

Note that parking areas count, not just buildings. A scheme that keeps the house clear of the line but puts the driveway and parking in it has not solved the problem.

The exception that matters for a rebuild

§25-7-93(A)(2) is the one to know if you own a house in the floodplain. An encroaching development application may be approved if it is:

"a new building for residential use that replaces an existing legally constructed building for residential use on the same property and that does not increase the number of legal dwelling units on the property."

Three conditions, all of which have to hold:

• the existing building was legally constructed;
• the replacement is residential replacing residential, on the same property; and
• it does not increase the number of legal dwelling units.

That last condition has a consequence people do not expect. HOME allows up to three units on most single-family lots — but taking that allowance on a floodplain lot forfeits this exception. One house replacing one house is permitted. One house becoming three is not, at least not by this route.

Two other general exceptions worth knowing: a parking area under 5,000 sq ft or an unoccupied structure under 1,000 sq ft, if the director finds no adverse effect on the floodplain or surrounding properties; and a building authorised by a waterway development permit issued before 25 September 1983.

The variance is real but demanding

§25-7-92(C) lets the director grant a variance — but only on seven findings, and the wording is conjunctive. All of them must hold:

1. finished floor elevation at least two feet above the 100-year floodplain;
2. normal access by direct connection with an area above the regulatory flood datum;
3. compliance with Chapter 25-12 Article 3 (Flood Hazard Areas);
4. the development compensates for the floodplain volume it displaces;
5. the development improves the drainage system beyond what §25-7-61 requires, demonstrated by a report certified by a Texas registered engineer;
6. the variance is required by unique site conditions; and
7. it causes no additional adverse flooding impact on other property.

Items 4 and 5 are the expensive ones — you are not merely avoiding harm, you are compensating for displaced volume and improving the system. And §25-7-92(D) requires the director to prepare written findings either way, which means the reasoning is on the record.

Residential infill gets a drainage concession

§25-7-67 was added by the 2025 infill ordinance and is genuinely useful. A residential infill development application is not required to comply with §25-7-61 (criteria for approval), §25-7-151 (stormwater conveyance and drainage facilities) or §25-7-152 (dedication of easements and rights-of-way) if either:

• the application is a resubdivision not exceeding 17,780 sq ft gross site area; or
• the applicant provides a drainage plan showing all stormwater runoff will discharge to an existing storm drainage system or into right-of-way.

Two conditions still attach. All proposed improvements must be outside the erosion hazard zone unless protective works are provided per the Drainage Criteria Manual; and the owner must provide an easement for stormwater flow to the limits of the 100-year floodplain.

On erosion hazard analysis generally, §25-7-32(B) exempts a minor, unoccupied residential appurtenance to a residential use, and development on a property separated from the triggering waterway by a public roadway — a road between you and the creek does real work here.

What the floodplain does to height

Two Subchapter F provisions key off it, both covered on the height page:

• the maximum height is 32 ft outside and 35 ft inside the 100-year floodplain; and
• where a site is in the floodplain, height is measured from natural grade rather than by the ordinary method.

Both depend on the §25-7-6 determination above — which is why "am I in the floodplain" has to be answered before the height question, not after.