Flood zones and elevation certificates: what to check before you build

FEMA's flood maps, called Flood Insurance Rate Maps (FIRMs), sort land into zones. The Special Flood Hazard Area is the land FEMA expects to be covered by a flood with a 1 percent chance of being equaled or exceeded in any given year, also called the base flood or 100-year flood. Zones A, AE, AH, AO, V and VE, among others, are all in it. [1]

AE is the one most people meet: the base floodplain where a base flood elevation is printed on the map. [3] VE is coastal land with the same 1 percent chance plus storm waves, which FEMA puts at a 26 percent chance of flooding over a 30-year mortgage. [2]

Zone X comes in two kinds. Shaded X (B on older maps) is the land between the 100-year flood and the 0.2 percent, or 500-year, flood. Unshaded X (C on older maps) is outside both. [1] Lower risk is not no risk, and some cities regulate the shaded X land too, as Houston does below.

Go to FEMA's Flood Map Service Center at msc.fema.gov, enter the street address and open the dynamic map to see the FIRMette, a printable piece of the official map for that spot. [6] FEMA's National Flood Hazard Layer viewer shows the same zones on a map you can pan. [8]

In Harris County, Texas, the Flood Control District runs MAAPnext, a restudy of the county's flood risk that also maps rainfall running off through streets and neighborhoods. Its interactive map lets you search an address, but the new maps are drafts: the site says they are not final and cannot be used for insurance or regulatory decisions, and that FEMA is expected to release preliminary FIRMs after its review. [13] Read a draft as a warning sign, and use the effective FEMA map for permits.

Your city or county floodplain administrator has the final word for a lot, and usually keeps elevation certificates on file for buildings already built. In Texas the Texas Water Development Board keeps a directory of them. [14]

The base flood elevation (BFE) is the height the water surface reaches in the 1 percent annual chance flood, printed on the map for zones such as AE and VE. [4] It is a height above a survey datum, not a depth above your ground, which is why it takes a surveyor to compare it with your lot.

Freeboard is extra height above the BFE built in as a margin of safety, for example 2 feet, and FEMA notes it results in significantly lower flood insurance rates. [5] Local rules set how much is required, and they vary a lot. Two examples from the Houston area, both stricter than building to the BFE:

City of Houston: under Chapter 19 of the city code, all new structures must be at least 2 feet above the 500-year elevation, or 3 feet for critical facilities and in the floodway. The rule covers the 500-year floodplain as well as the 100-year floodplain and the floodway. [11]

Unincorporated Harris County (outside city limits): for a permit in an A zone, a V zone, the floodway or below the 100-year flood elevation, the top of the subfloor of the lowest habitable floor must be 24 inches or more above the 500-year flood elevation, or 12 inches above the crown of the nearest public street, whichever is higher. In a floodway, the bottom of the lowest supporting member must be 36 inches above the 500-year elevation. Water heaters, air conditioning and electrical panels go at least 24 inches above the 500-year elevation too. [10]

Those are the rules of one city and one county, and rules change. Anywhere else, ask your own floodplain administrator what the required elevation is before anything is designed.

An elevation certificate is FEMA's form (FEMA Form FF-206-FY-22-152, formerly 086-0-33) for recording how high a building sits. Communities use it to show that new buildings in the Special Flood Hazard Area were elevated as their ordinance requires, and owners use it to get flood insurance. [7]

The building elevations on it come from a survey, and that part is certified by a land surveyor, engineer or architect, with their seal when a surveyor or engineer completed it. [8]

On a new house it is often needed more than once. Harris County asks for one signed and sealed by a registered surveyor or engineer before framing, and a final one when the house is ready to live in. [10] The City of Houston asks for one based on the construction drawings at plan review, then for its inspections while the building is under construction and again when it is finished. [12]

Keep the final certificate. It is the document an insurance agent asks for.

Homes in high-risk flood areas with mortgages from government-backed lenders are required to carry flood insurance, and most homeowners policies do not cover flood. [9]

Through the National Flood Insurance Program, residential building coverage goes up to $250,000 and contents up to $100,000, and a new policy typically has a 30-day waiting period, with exceptions such as a purchase a lender requires. [9] The elevation certificate is one of the documents a policy can be rated from, which is one reason freeboard pays back. [5] [7]

Ask an agent for a quote on the lot before you buy it, at the floor height you are planning.

A required floor height is a foundation question first. Unincorporated Harris County does not allow fill to raise a building in the 100-year floodplain: the house goes on an open foundation such as piers, or on continuous foundation walls with properly sized openings, and every foundation there must be designed by a registered professional engineer. [10] Openings in enclosed areas below the flood level must let water in and out, and a licensed architect or engineer certifies them. [10]

Every plan on this site lists its foundation options on the plan page, and the plans for sale offer a post tension slab, pier and beam, or a crawlspace. Those are starting points for your engineer, not a design for your lot. If your lot needs the floor several feet up, plan on a foundation designed by a licensed engineer for your soil report and your lot elevation.

A raised floor also changes the house around it: more steps at every door, a stair or ramp down to the garage, and a taller house overall. Plan pages give ridge height from the top of the foundation, so add your floor height before you check it against a height limit in deed restrictions or zoning.

Lot fit matters too. A raised house still needs room for its stairs, and part of a lot in the floodplain may be harder to build on. The lot fit guide explains how the site checks width and setbacks.

Which flood zone is the lot in on the effective map, and does a draft map such as MAAPnext show it differently?

What elevation does the floor have to meet here, under which rule, and who confirmed it with the floodplain administrator?

Which surveyor will do the elevation certificates, and at which stages does this permit office want them?

Which foundation do you propose for that height, and which licensed engineer will design it?

How much of the budget goes to the foundation and site work at this height, compared with the same house on higher ground?