A shaded flood zone on the survey, a note in the title report, or a lender asking for an elevation certificate: however the floodplain comes up, the first question is whether the project can still be built. In Colorado it usually can. What changes is the number of agencies involved, the order the approvals have to happen in, and the schedule. Projects get hurt when the floodplain is found late. They go fine when it is treated as a design input from the first sketch.
Floodway vs. Flood Fringe
The mapped 100-year floodplain (the area with a 1 percent chance of flooding in any year) usually has two parts. The floodway is the channel and the strip beside it that has to stay open to carry the flood. The flood fringe is the rest of the mapped area, where water spreads out and slows down.
Building in the fringe is common. Structures are elevated or floodproofed, and fill is placed so it doesn’t push water onto neighbors. Building in the floodway is a different matter. Any fill or structure there has to be shown by hydraulic modeling to cause no rise in the flood elevation, which rules out most buildings. If the project touches the floodway, that fact drives the rest of the design.
Colorado’s Rules Are Stricter Than FEMA’s
Colorado’s floodplain rules, adopted by local jurisdictions, go beyond FEMA’s minimums in two ways that come up on almost every project. New and substantially improved buildings need their lowest floor, or floodproofing for nonresidential buildings, at least one foot above the base flood elevation, where FEMA’s minimum is at the elevation. And floodways in Colorado are drawn to allow no more than half a foot of rise from encroachment, compared with FEMA’s one foot, so the floodway is wider and the fringe where building is easier is narrower. Some local codes go further still.
The Approvals and Who Issues Them
| Approval | Issued by | When you need it |
|---|---|---|
| Floodplain development permit | Local floodplain administrator | Any development in the mapped floodplain, including buildings, additions, fill, grading, and paving. |
| No-rise certification | Engineer, reviewed locally | Any work in the floodway, backed by hydraulic modeling. |
| CLOMR | FEMA, after local review | Before construction, when the project will change the floodplain. |
| LOMR | FEMA | After construction, to revise the map based on as-built conditions. |
| LOMA | FEMA | When a building or lot sits naturally above the base flood elevation but was mapped into the floodplain. |
| Section 404 permit | U.S. Army Corps of Engineers | Fill or structures in a stream, wetland, or other federally regulated water. |
| Elevation certificate | Licensed surveyor or engineer | New and substantially improved buildings in the floodplain, and for flood insurance. |
Floodplain approvals usually run alongside the site’s other stormwater requirements. See detention pond requirements for commercial sites.
CLOMR vs. LOMR
When a project changes the floodplain itself, by filling part of it, improving the channel, or replacing a culvert, the FEMA map has to change too. That happens in two steps. A CLOMR, a Conditional Letter of Map Revision, is FEMA’s review of the proposed change before construction. It confirms that the design, if built as shown, will change the floodplain the way the model says. A LOMR, a Letter of Map Revision, follows construction and revises the effective map based on the as-built survey. The CLOMR is what lets the project proceed with confidence. The LOMR is what makes the new map official, and it is what changes flood insurance requirements for the properties affected.
Army Corps 404 Permits
Separately from the floodplain, placing fill or structures in a stream, wetland, or other water under federal jurisdiction requires a Section 404 permit from the Army Corps of Engineers. Many small projects fit a general, nationwide permit. Larger impacts need an individual permit, which takes much longer. Which streams and wetlands are federally regulated narrowed after a 2023 Supreme Court decision, so the first step is often a determination of whether the feature is regulated at all.
Existing Buildings and the 50 Percent Rule
For an existing building in the floodplain, a renovation or addition that costs half or more of the building’s market value is a substantial improvement, and the whole building then has to meet current floodplain standards. That can turn a remodel into an elevation project. Know the number before scoping the renovation.
How Long It Takes
A project that stays out of the floodplain, with the boundary confirmed by survey, adds almost nothing. A project in the fringe that elevates its building adds a floodplain permit and an elevation certificate, measured in weeks. A project that changes the floodplain adds the CLOMR: hydraulic modeling, local review, then FEMA review, usually several months. An individual 404 permit can add many more months, and these processes often have to run in sequence rather than in parallel. That sequencing is why finding the floodplain late is so expensive.
On these projects we do the site drainage engineering, and we coordinate the floodplain permit, the FEMA map revision, and the Army Corps permitting with the agencies and the specialists each process needs, so the approvals line up with the design.
Common Questions
Can you build in a floodplain in Colorado?
In the flood fringe, usually yes, with a local floodplain development permit and a building elevated or floodproofed at least one foot above the base flood elevation. In the floodway, rarely, because the project has to show no rise in flood levels.
What is the difference between a CLOMR and a LOMR?
A CLOMR is FEMA’s review of a proposed change to the floodplain before construction. A LOMR revises the effective flood map after construction, based on the as-built survey.
How high does a building have to be above the floodplain in Colorado?
Colorado’s rules generally require the lowest floor of new and substantially improved buildings to be at least one foot above the base flood elevation. Some local codes require more.