Letter of Map Revision (LOMR): When Flood Maps Change

A Letter of Map Revision (LOMR) is an official amendment to a Flood Insurance Rate Map (FIRM) issued by the Federal Emergency Management Agency (FEMA), reflecting physical changes to a community's floodplain. LOMRs carry direct consequences for flood insurance obligations, property valuations, and lender requirements. Understanding when a LOMR applies, how the process unfolds, and how it differs from related instruments such as a Letter of Map Amendment (LOMA) is essential for property owners, local governments, and lenders navigating the National Flood Insurance Program.

Definition and Scope

A LOMR is a formal letter from FEMA that officially revises the current effective Flood Insurance Rate Map. Unlike a LOMA — which corrects an error by showing that a specific structure or parcel was inadvertently included in a Special Flood Hazard Area (SFHA) — a LOMR reflects a physical change to the landscape or hydraulic conditions that actually alters flood risk. These changes can affect entire communities, not just individual properties.

LOMRs operate under the authority of the National Flood Insurance Act of 1968 and are administered through FEMA's Flood Map Service Center. The process is governed by Title 44 of the Code of Federal Regulations (44 CFR Parts 65 and 70), which establishes the documentation standards and review procedures communities and requestors must follow (eCFR, 44 CFR Part 65).

The scope of a LOMR is community-wide in nature. Once FEMA issues and effective-dates a LOMR, the revised map supersedes the prior FIRM for all regulatory and insurance purposes. Communities participating in the National Flood Insurance Program (NFIP) are required to adopt and enforce the revised floodplain management standards reflected in the new map.

How It Works

The LOMR process follows a structured sequence of steps that begins with the triggering physical change and ends with FEMA publishing a revised effective FIRM.

1. Triggering event identification. A physical change — such as a completed flood control project, channel modification, or significant fill activity — is documented by a licensed engineer or community official.
2. Preparation of technical documentation. The requester (typically a local government or developer) compiles an engineering report that includes hydraulic and hydrologic analyses, certified topographic data, and project-as-built drawings. FEMA's MT-2 application forms are used for this submission.
3. Submission to FEMA. The package is submitted to FEMA's Flood Map Service Center. FEMA has established a standard review period of approximately 60 days for most LOMR requests, though complex cases can require longer review (FEMA Flood Mapping Program).
4. FEMA technical review. FEMA's contractors and engineers evaluate the hydraulic models, verify compliance with 44 CFR Part 65 standards, and may request supplemental data.
5. Community notification and comment period. Affected communities and property owners receive notice. A 90-day appeal period applies when the LOMR proposes changes to Base Flood Elevations (BFEs).
6. Issuance and effective date. FEMA issues the final LOMR letter, which carries an effective date — typically 6 months after issuance — allowing mortgage lenders, insurers, and communities time to implement the changes.

The flood maps and FIRM explained resource provides additional context on how FIRMs function as the underlying regulatory documents that LOMRs revise.

Common Scenarios

LOMRs arise under three primary categories of physical change:

Structural flood control projects. When a community completes a certified levee, floodwall, detention basin, or channel improvement, properties previously mapped within the 100-year floodplain may be removed from the SFHA. FEMA requires that levees meet the design and certification standards in 44 CFR Part 65.10, including the ability to withstand the 1-percent-annual-chance flood event without overtopping or structural failure.

Channelization and watercourse modifications. Rerouting, widening, or deepening a stream channel changes the flood conveyance characteristics of that watercourse. Engineering analysis must demonstrate the revised channel geometry and updated hydraulic model outputs.

Large-scale fill and grading projects. When significant fill raises the grade of land above the BFE across a broad area — such as a subdivision development or commercial site — a LOMR can remove those parcels from the mapped floodplain. This is distinct from a single-lot fill scenario, which would typically be addressed through a LOMA or LOMR-F (Letter of Map Revision Based on Fill) process.

A related instrument worth distinguishing is the LOMR-F, a subcategory specifically for fill-based elevation changes on individual lots or small areas. LOMR-F requests use a different application pathway and are evaluated under 44 CFR Part 70, whereas standard LOMRs covering broader hydraulic changes use 44 CFR Part 65.

Changes to base flood elevation are among the most significant outputs of a LOMR, as they cascade directly into flood insurance premium calculation and lender escrow determinations.

Decision Boundaries

Not every map change qualifies for — or requires — a LOMR. The boundaries that determine applicability involve the nature of the change, the scale of affected area, and whether the change reflects physical reality or administrative correction.

LOMR vs. LOMA: A LOMR is warranted when a physical change has genuinely altered flood risk for an area. A LOMA is appropriate when a property was incorrectly included in an SFHA based on mapping error, without any physical modification to the site. The LOMA guide covers that instrument separately.

LOMR vs. Map Modernization (Countywide Remapping): FEMA periodically undertakes full countywide or statewide remapping efforts that supersede existing FIRMs entirely. A LOMR is a targeted, site-specific or project-specific revision rather than a comprehensive remap. When a full remap is underway for a community, pending LOMR requests may be absorbed into the broader mapping process.

Insurance obligation consequences: Removal from an SFHA via a LOMR relieves property owners of the mandatory flood insurance requirement attached to federally backed mortgages under the Flood Disaster Protection Act of 1973. However, voluntary coverage through the NFIP or private flood insurance options remains available, and flood insurance for low-to-moderate risk zones is often significantly less expensive than high-risk zone policies.

Effective date compliance: Communities that fail to adopt the revised floodplain management ordinance reflecting a new LOMR within the established timeframe risk probation or suspension from the NFIP under 44 CFR Part 59, which would strip residents of access to federally backed flood insurance entirely.

References

• FEMA Flood Map Service Center
• FEMA — National Flood Insurance Program Overview
• FEMA Flood Maps and Mapping Program
• eCFR — 44 CFR Part 65: Identification and Mapping of Special Flood Hazard Areas
• eCFR — 44 CFR Part 70: Procedure for Map Correction
• eCFR — 44 CFR Part 59: General Provisions, NFIP
• FEMA MT-2 Application Forms for Map Amendments and Revisions
• National Flood Insurance Act of 1968 — FEMA Reference
• Flood Disaster Protection Act of 1973 — Congressional Reference via FEMA