What the rule says
The rule comes from the National Flood Insurance Program. Federal regulations (44 CFR 59.1) define a substantial improvement as any reconstruction, rehabilitation, addition or other improvement whose cost equals or exceeds 50% of the structure's market value before construction starts. Substantial damage is damage of any origin where restoring the building would cost 50% or more of its pre-damage market value. A substantially damaged home is treated as substantially improved no matter how much repair work is actually done.
Two kinds of work are excluded under the federal definition: work to correct existing health, sanitary or safety code violations that the code official has identified, limited to the minimum needed, and alterations to a designated historic structure that do not end its historic designation.
The math
Cost of all improvements or repairs to the structure ÷ market value of the structure (land excluded) × 100. At 50% or more, the project is substantial.
How market value is set
Every jurisdiction in this guide starts from the building value on the county property appraiser's record, with the land value left out. Collier County's appraiser covers Naples, unincorporated Collier and Marco Island; Lee County's covers Bonita Springs and Estero.
- City of Naples uses the Collier County Property Appraiser's assessed building value, which excludes land, pool and spa, or a Florida-licensed appraiser's depreciated replacement cost dated within six months of permit submittal. An insurance appraisal that leaves out depreciation may not be accepted.
- Collier County points owners to the improved values on the Collier County Property Appraiser's site and requires its substantial improvement affidavit packet and cost worksheet.
- Bonita Springs and Estero use the Lee County Property Appraiser's building value, or a private appraisal of the building's depreciated value.
- Marco Island applies the rule to the structure's market value. Ask the City's Floodplain Management office (239-389-3926) which valuation sources it accepts before you rely on one.
Florida's Division of Emergency Management told communities in January 2024 that any adjustment to assessed values must come in writing from the county property appraiser, and that communities must not write a fixed adjustment factor into their codes.
What counts toward the 50%
The cost estimate has to cover all the work on the structure, not just structural work. The Village of Estero's published list is a good guide to what local officials typically include and exclude:
- Included: structural work, finishes, built-in systems (electrical, plumbing, HVAC), demolition labor, contractor overhead and profit, sales tax, and the market value of owner or volunteer labor and donated materials.
- Usually excluded: debris cleanup, permit, plan and inspection fees, plans and surveys, landscaping, fences, driveways, pools, sheds, detached garages and plug-in appliances.
The City of Naples requires a licensed contractor to prepare and sign the estimate along with an affidavit. Collier County says owners cannot make incremental improvements to stay under the line and must provide a full estimate even when work is phased.
What happens over 50%
The whole building must meet current flood rules. Across all four communities that means the lowest floor elevated to at least one foot above base flood elevation (the Florida Building Code minimum), or to the local design flood elevation if that is higher, and space below that level limited to parking, storage and access. Estero and Lee County also require equipment, bathrooms and laundry to be elevated. In coastal V zones, the measurement is to the bottom of the lowest horizontal structural member, which usually means building on pilings.
What changed in 2025: no more lookback periods
Several local governments used to add up improvement costs over a period of years, often five, so that a series of smaller projects could trigger the rule together. Florida's SB 180, signed June 26, 2025, created section 163.31795 of the Florida Statutes, which says a local government in the flood insurance program may not adopt or enforce an ordinance that includes a cumulative substantial improvement period. The City of Naples had already ended its one-year lookback in December 2024.
Some local pages still show the older five-year rule (Bonita Springs Ordinance 22-05, Lee County Ordinance 22-30, an Estero handout and an older Marco Island flyer). If you see one, ask the building department to confirm how it is applying the rule today.
The same law requires every Florida county and city to offer an online way to receive and review substantial damage and substantial improvement letters by May 1, 2026.
Who to call
| COMMUNITY | OFFICE | PHONE |
|---|---|---|
| City of Naples | Building Department, Floodplain Review, 295 Riverside Circle | 239-213-5020 |
| Unincorporated Collier (incl. Pelican Bay, North Naples outside the City) | Collier County Floodplain Management | 239-252-2942 |
| City of Marco Island | Floodplain Management, 50 Bald Eagle Drive | 239-389-3926 |
| City of Bonita Springs | Community Development, 9220 Bonita Beach Rd SE, Suite 111 | 239-444-6150 |
| Village of Estero | Community Development, 9401 Corkscrew Palms Circle | 239-221-5036 |
Three worked examples (illustrations, round numbers)
Example 1: a remodel that crosses the line
Building value from the property appraiser: $400,000 (land excluded). Planned whole-home remodel: $210,000. $210,000 ÷ $400,000 = 52.5%. That is a substantial improvement, so the home must be brought into full compliance, including elevation. Trimming the scope to $195,000 gives 48.75%, under the line.
Example 2: why the market value source matters
The appraiser's building value is $250,000 and the planned work is $150,000, which is 60%. A Florida-licensed appraiser's depreciated replacement cost comes in at $340,000, and the same work is 44%. Where the jurisdiction accepts a private appraisal, it can change the answer. It has to follow the local rules on method and date.
Example 3: storm repairs plus upgrades
Pre-damage building value: $300,000. Repairs back to pre-storm condition: $140,000, which is 46.7%. Adding a $20,000 kitchen upgrade to the same project brings it to $160,000, or 53.3%, and the home becomes substantially improved. Splitting the upgrade onto a second permit does not get around it in Collier County, which requires the full estimate.
Sources
- 44 CFR 59.1 definitions: ecfr.gov/current/title-44/chapter-I/subchapter-B/part-59/subpart-A/section-59.1
- FEMA P-758, Substantial Improvement/Substantial Damage Desk Reference
- Florida Building Code 8th Ed. (2023) flood provisions R322 (FDEM excerpt): floridadisaster.org
- Section 163.31795, F.S. (SB 180, 2025): flsenate.gov/Laws/Statutes/2025/0163.31795
- FDEM market value guidance, Feb 2022 and Jan 2024: floridadisaster.org
- City of Naples floodplain pages: naplesgov.com/building/page/development-floodplain ; /flood-mitigation-guidelines-regulations-property-owners
- City of Naples Ordinance 2024-15508 (ends one-year lookback)
- Collier County SI/SD analysis and Building Within the Floodplain: collier.gov/Business-Resources/Floodplain-Management
- City of Marco Island floodplain management: cityofmarcoisland.com/building/page/city-marco-island-floodplain-management
- City of Bonita Springs post-storm permitting guide and Ordinance 22-05: cityofbonitasprings.org
- Village of Estero SI/SD page and handout: estero-fl.gov/substantial-damage-substantial-improvement
- Lee County DCD improvements page and Ordinance 22-30: leegov.com/dcd/flood
This guide is general information, not legal or engineering advice. Rules change; confirm with your local building department before you plan.