What Is a LOMR? Letter of Map Revision Explained

A Letter of Map Revision, or LOMR, is FEMA's official revision to an effective flood map, issued when physical changes or new and better data justify updating the map itself rather than a single property. A LOMR can move flood zone boundaries, change Base Flood Elevations, or redraw the floodway, and once it takes effect, it applies to everyone in the affected area, not just one parcel. Common reasons FEMA issues a LOMR include new hydrologic or hydraulic data, the placement of fill, channel or drainage improvements, or the construction of a levee or other flood control structure.

Here is what a LOMR actually changes, how it differs from the conditional version issued before a project is built, and how a map change can connect to the standard NFIP waiting period.

What a LOMR Actually Changes

Unlike a determination that applies to a single property, a LOMR revises the flood map itself for the area it covers. Depending on what triggered the revision, a LOMR can:

• Move the Special Flood Hazard Area boundary, taking some properties out of the high-risk zone and, in some cases, bringing others into it.
• Change the Base Flood Elevation for an area, based on new hydrology or hydraulic modeling.
• Redraw the floodway, the channel and adjacent land that must be kept clear to carry flood flows without raising flood heights.

Because a LOMR changes the official map, it affects every property within the revised area, and it becomes the new basis for flood zone determinations, Base Flood Elevations, and mandatory flood insurance requirements going forward.

FEMA issues a LOMR when it has physical changes or new data that justify updating the community's flood map. The most common triggers include new or improved hydrologic and hydraulic data, such as updated rainfall or stream-flow studies; fill placed to raise land above the Base Flood Elevation across a broader area, rather than a single lot; channel, drainage, or culvert work that changes how water moves through an area during a flood; and construction of a levee, floodwall, or other flood control structure that is finished, certified, and recognized by FEMA as providing protection from the 1% annual chance flood.

LOMR vs. LOMA: Two Different Kinds of Map Change

A LOMR and a Letter of Map Amendment (LOMA) are both FEMA determinations that change how a property is classified on the flood map, but they work very differently. A LOMA corrects the map for a single property that was already on natural high ground above the Base Flood Elevation — it is narrow, property-specific, and does not involve construction or fill. A LOMR, by contrast, is a revision to the map itself, driven by physical changes or new data that affect an area, and it can change Base Flood Elevations, zone boundaries, or the floodway for everyone within that area, not just one lot.

What Is a CLOMR, and How It Differs from a LOMR

A Conditional Letter of Map Revision, or CLOMR, is the conditional version of a LOMR. FEMA issues a CLOMR before a proposed project — such as a levee, fill project, or channel improvement — is actually built. A CLOMR tells a property owner, developer, or community what FEMA's determination would be if the project is completed as proposed, based on the plans submitted.

A CLOMR does not revise the effective flood map. The map only changes once the project described in the CLOMR is actually constructed, and FEMA issues the final LOMR based on as-built conditions. Until that final LOMR is issued and takes effect, the current effective map remains in force, along with any flood insurance requirements tied to it.

How a LOMR Affects Your Flood Insurance Requirement

Because a LOMR revises the effective flood map, it can change whether a property is inside or outside the Special Flood Hazard Area, which in turn affects the federal mandatory flood insurance purchase requirement tied to a mortgage. A property removed from the Special Flood Hazard Area by a LOMR is no longer subject to that mandatory requirement going forward. A property that a LOMR newly brings into the Special Flood Hazard Area can become subject to it.

As with any map-based determination, the LOMR has to be in effect and reflected in the records lenders rely on before the requirement changes in practice.

The LOMR and the NFIP Waiting Period Exception

The standard NFIP flood policy has a 30-day waiting period before new coverage takes effect. One of the recognized exceptions to that waiting period applies when a flood insurance purchase is required as the result of a map revision, such as a LOMR, that newly places a building in the Special Flood Hazard Area. In that situation, the standard waiting period exception tied to a map change can apply, rather than the full 30 days otherwise required for a new policy.

Common Mistakes with LOMRs

• Confusing a LOMR with a LOMA. A LOMR revises the map for an area; a LOMA corrects the map for one property already on natural high ground.
• Treating a CLOMR as if the map has already changed. The effective map does not change until the project is built and FEMA issues the final LOMR.
• Assuming a LOMR only removes properties from the Special Flood Hazard Area. A LOMR can also bring properties into the zone, depending on what the new data or physical change shows.
• Dropping flood insurance before the final LOMR is issued and in effect. A CLOMR or an in-progress project is not the same as a completed map revision.

Pro Tips from Sterling Flood Underwriters

Keep a copy of the LOMR, not just the CLOMR, with your property records. Lenders and future insurers will want to see the final determination, not the conditional one.

Check whether a LOMR affects your Base Flood Elevation, not just your zone letter. A revised BFE can matter for underwriting even if the zone designation itself looks similar.

Get a Quote

Whether a property's flood zone comes from the current effective map or a recent LOMR, Sterling can quote flood coverage either way, without requiring an elevation certificate. Get a flood insurance quote from Sterling Flood.

Frequently Asked Questions

What does LOMR stand for?

Letter of Map Revision. It is FEMA's official revision of the effective flood map, based on physical changes or new data such as updated hydrology, fill, channel work, or a completed flood control structure.

How is a LOMR different from a LOMA?

A LOMR revises the flood map itself for an area, which can change Base Flood Elevations, zone boundaries, or the floodway for everyone in that area. A LOMA only corrects the map for a single property already on natural high ground.

What is a CLOMR?

A Conditional Letter of Map Revision is the conditional version FEMA issues before a proposed project is built. It shows what FEMA's determination would be if the project is completed as proposed, but it does not revise the effective map until construction is finished and a final LOMR is issued.

Can a LOMR affect my flood insurance waiting period?

Yes, in certain cases. When a map revision such as a LOMR newly places a building in the Special Flood Hazard Area, the standard 30-day NFIP waiting period has a recognized exception that can apply to that situation.