Austin drainage and detention rules: what a development application must show

Last verified August 5, 2026 against the Austin Land Development Code · KiliGray Homes

A development application cannot be approved unless it controls the two-year peak flow on-site, shows capacity for the design flood, causes no additional adverse flooding or erosion on other property, and keeps improvements out of the erosion hazard zone.

Applies to: Development applications in Austin. Residential infill has a specific exemption from several of these requirements under §25-7-67.

Two different requirements, and people conflate them

The code and the manual are doing different jobs here, and reading only one of them gives the wrong answer.

The code's approval criterion. §25-7-61(A)(5)(c) requires a development application to include on-site control of the two-year peak flow, as determined under the Drainage Criteria Manual and Environmental Criteria Manual. That is the threshold test for approving the application.

The manual's pond design criteria. Where you actually build a detention pond, DCM §8.3.2(A) requires it to reduce post-development peak rates of discharge to existing pre-development rates for the 2-, 10-, 25- and 100-year storm events, at each point of discharge from the site.

So "detention in Austin is just the two-year" is wrong as a description of pond design, even though the two-year is the figure the code names. If a pond is required, it is sized across four events, not one.

§25-7-61(B) does allow off-site control of the two-year flow instead of on-site, but only if doing so causes neither an adverse water quality impact from increased in-stream peak flow, nor streambank erosion.

The other four things an application must show

Alongside the two-year control, §25-7-61(A) requires all of the following. They are cumulative — the application may not be approved unless every one is satisfied.

Capacity for the design flood, as determined under the Drainage Criteria Manual.
Each improvement strong enough to resist external pressure from earth or building, and internal pressure or abrasion from water or debris.
Grades that will not pool stagnant water — expressly excluding variable pools in creek beds resulting from natural channel design.
Erosion control, temporary and permanent, sufficient to minimise siltation of the waterway, under the Environmental Criteria Manual.

And within subsection (5), three more alongside the two-year control: no additional adverse flooding impact on other property; no additional erosion impacts on other property; and all improvements outside the erosion hazard zone unless protective works are provided. Plus a preservation duty — to the greatest extent feasible, preserve the natural and traditional character of the land and waterway within the 100-year floodplain.

Residential infill is exempted from much of this. Under §25-7-67, an application does not have to comply with §25-7-61, §25-7-151 or §25-7-152 if it is a resubdivision not exceeding 17,780 sq ft gross site area, or if the applicant provides a drainage plan showing all runoff discharging to an existing storm drainage system or into right-of-way. That is the single most useful provision in the chapter for small infill, and it is covered on the floodplain page.

If you do build a pond

Three criteria from DCM Section 8 that shape the design:

Four storm events. Post-development peaks reduced to pre-development peaks for the 2-, 10-, 25- and 100-year events at each discharge point. Off-site areas draining to the pond are assumed to stay in their existing condition, and if off-site flows pass through it the outlet must safely pass 100-year fully developed off-site flows.
24-hour drawdown. Maximum retention time may not exceed 24 hours from peak storage to complete emptying. Where a facility is required to drain more slowly, only the volume in a water quality control available after 24 hours may be credited toward detention.
64 acres makes it regional. Any pond with a drainage area larger than 64 acres is classified as a regional pond, and its performance criteria are reviewed project by project by Watershed Protection on a preliminary engineering study by a Texas-licensed engineer.

That 64-acre figure is the same threshold that makes a waterway a regulated minor waterway — worth noticing, since it appears in two unrelated parts of the regulations.

Paying instead of detaining: the Regional Stormwater Management Program

The DCM sets out a route that is easy to miss and can remove the pond entirely. The Regional Stormwater Management Program lets a developer participate in lieu of constructing on-site detention, provided the development will produce no additional adverse flooding impact on nearby and downstream properties.

Participation is ranked by preference, and payment is the last of four:

improving part of the downstream drainage system, or stormwater control measures in the immediate vicinity of the project;
off-site detention;
dedicating easements or donating land for regional improvements; then
payment in lieu of detention.

The payment is calculated on total site area less dedicated floodplain easements, drainage easements containing the 100-year floodplain, permanent retention or detention facilities, and other areas undevelopable under City code — so the deductions can be substantial on a constrained site.

Two practical points. Not every watershed participates — there is a published Austin Watersheds List — and RSMP applications are reviewed during the site plan or subdivision review and will not be approved out of cycle. It is a thing to raise at submittal, not afterwards.

Water crossing your land is your responsibility

§25-7-151(A) is broader than owners expect. The owner or developer is responsible for conveying all stormwater flowing through the property — including water that:

is directed to the property by other developed property; and
naturally flows through the property because of the topography.

So "the neighbour's runoff is the neighbour's problem" is not the rule. If it crosses your land, conveying it is your obligation when you develop. And §25-7-151(B) adds that future upstream development must be accounted for, not just today's flows.

Two more that carry cost. Where a drainage facility is required along a shared property line, the owner who develops is responsible for the facility on either side of that line — and for dedicating or obtaining any easement or right-of-way needed for it. And §25-7-151(F) requires adequate off-site improvements to accommodate the full effects of the development. The city may help acquire a necessary property interest, but only if the owner certifies by affidavit that a genuine attempt failed and guarantees the entire acquisition cost.

Who maintains a detention basin — and the three-unit trap

§25-7-153 splits basins into two regimes, and the dividing line is worth reading carefully:

Residential development means development of two dwelling units or less per lot.
Commercial development means all development other than residential development.

Read together, those definitions put a three-unit build on the commercial side — which matters, because HOME made three units routine on ordinary single-family lots. The section dates from 2013 and predates HOME, so this may be a consequence of the definitions rather than a deliberate choice. We report what the section says; we have not confirmed how the City applies it, and would ask rather than assume on a live project.

If it is a commercial basin, the record owner maintains it to Drainage Criteria Manual standards whether or not it sits on the same property — and must prove a right of access if it does not. Where one basin serves several developments, all the record owners are jointly and severally responsible. The City inspects non-subsurface commercial basins at least every three years, with a re-inspection fee if one fails. A subsurface commercial basin needs a maintenance plan and an annual report from a registered engineer.

If it is a residential basin, the owners maintain it until the City accepts it — and the City accepts it only on determining that it meets all Drainage Criteria Manual requirements. Until that happens the obligation stays with you.

Drainage studies, and not blocking the creek

§25-7-31 lets the director require a drainage study, at the owner's expense, for the total area to be developed. The sting is in subsection (D): if a study is required, the City may not accept for review a development application for any portion of the proposed development until the director has received it. It is a gate on submission, not a condition of approval — so it lands earlier in the programme than people plan for.

And two short prohibitions worth knowing. §25-7-3: unless authorised by an approved development application, a person may not place an obstruction in a waterway. §25-7-4 puts a continuing duty on whoever controls land a waterway crosses to keep it free of unauthorised obstruction. §25-7-5 then declares standing water caused by an unauthorised obstruction a nuisance.

Between them these cover the fill, the retaining wall and the piled brush that show up in creek-side back gardens, and they apply to the person in control of the property whether or not they put the obstruction there.

What this means on a normal infill job

On most of our sites the §25-7-67 infill route is the answer — a drainage plan showing runoff going to an existing storm system or into the right-of-way, and the heavier §25-7-61 machinery does not apply. Establishing that early is worth real money.

Where it does not apply, the two-year peak flow control is the design driver, and it is much less onerous than clients fear when they hear "detention". The conversation that actually gets difficult is §25-7-151 — upstream water crossing the site, and who pays to carry it.

The three-unit basin definition is one we would raise with Watershed Protection rather than resolve from the text. If a three-unit project needs a basin at all, whether it is treated as commercial changes who inspects it and for how long the obligation runs.

Common questions

Does Austin require detention on a new house?

Only where §25-7-61 applies. It requires on-site control of the two-year peak flow — not the 100-year. Many residential infill projects are exempt under §25-7-67 if runoff discharges to an existing storm drainage system or the right-of-way, or the application is a resubdivision of 17,780 sq ft or less.

What storm event does Austin detention have to control?

Two answers, at two levels. The code criterion for approving an application is on-site control of the two-year peak flow (§25-7-61(A)(5)(c)). But where a detention pond is built, Drainage Criteria Manual §8.3.2(A) requires it to reduce post-development peaks to pre-development peaks for the 2-, 10-, 25- and 100-year events at each discharge point.

Can I pay instead of building detention in Austin?

Potentially, through the Regional Stormwater Management Program, if the development produces no additional adverse flooding impact downstream. Payment is the last of four preferred participation options, and applications are reviewed during site plan or subdivision review rather than out of cycle.

How fast must an Austin detention pond drain?

Within 24 hours from peak storage to complete emptying, under DCM §8.3.2(C). Where a facility must drain more slowly, only the volume in a water quality control available after 24 hours counts toward detention.

Who is responsible for stormwater flowing onto my property in Austin?

When you develop, you are. §25-7-151(A) makes the owner or developer responsible for conveying all stormwater flowing through the property, including water directed there by other developed property and water that naturally flows through because of topography.

Who maintains a detention basin in Austin?

For a commercial basin, the record owner — jointly and severally with others where one basin serves several developments. For a residential basin, the owners maintain it until the City accepts it, which happens only when it meets all Drainage Criteria Manual requirements.

Is a three-unit project residential for Austin detention basin rules?

On the face of §25-7-153, no. It defines residential development as two dwelling units or less per lot and commercial development as everything else, so a three-unit build falls on the commercial side. The section predates HOME, so confirm the current practice with Watershed Protection.

When does Austin require a drainage study?

The director may require one at the owner's expense for the total area to be developed. Under §25-7-31(D) the City may not accept any part of the development application for review until the required study has been received.

Working out what you can build?

We build in Central Austin and deal with this code on every project. If you own a lot and want a straight answer about what fits on it, we're happy to take a look.

Talk to us about your lot

We are homebuilders, not the City of Austin. This page explains how we read the code in practice — it is not legal advice and it is not an official determination. Always confirm with Austin Development Services before you rely on it for a project. The authoritative text is the Austin Land Development Code; permitting questions go to Austin Development Services Department.