Flood Zone & Floor Height Checker
Pick your FEMA zone and project, enter the flood elevations from your map and the ground height at the house, and see how high the lowest floor has to be and how far that is above grade. The 3D lot shows the 100-year and 500-year water, the required floor line and the fill pad or piers.
- Required floor—
- Raise vs normal slab—
- 100-yr / 500-yr—
- Foundation—
Drag to turn · scroll or pinch to zoom · blue is the 100-year water, light blue the 500-year, green the required floor · heights are above your existing grade
How the required elevation is worked out
City of Houston (Chapter 19, effective September 1, 2018): the City's guideline summary sets the lowest floor for new residential construction in both the 100-year and 500-year floodplains at the 500-year elevation plus 2 ft. A residential addition over 1/3 of the footprint also gets 500-year plus 2 ft in both floodplains. A residential addition of 1/3 of the footprint or less is 100-year plus 1 ft in the 100-year floodplain and has no requirement in the 500-year floodplain. Substantial improvement is 500-year plus 2 ft in the 100-year floodplain and has no requirement in the 500-year floodplain. Fill placed below the 500-year elevation must be offset with compensating storage.
Unincorporated Harris County: the County's floodplain regulations (amended effective January 1, 2018) require, for a Class II permit (land in an A zone or below the 100-year elevation), that the top of the subfloor of the lowest habitable floor be 24 in or more above the 500-year elevation, or 12 in above the crown of the nearest public street, whichever is higher. In a floodway or V zone the bottom of the lowest supporting member must be 36 in above the 500-year elevation. We applied this to new construction and substantial improvement. The text we could read does not give a clear number for additions or for lots only in the 500-year floodplain, so the tool tells you to check with Harris County Engineering.
Everywhere else: the federal minimum for the National Flood Insurance Program is that new residential construction and substantial improvements in A, AE and AH zones have the lowest floor at or above the base flood elevation. Many communities require extra height (freeboard), so ask your local floodplain administrator.
What the 3D lot shows
Everything is measured up from your existing grade. The darker blue plane is the 100-year water level (BFE), the lighter one the 500-year level, and the green plane is the required floor. The house sits on a slab at your normal slab height when no raise is needed, on a fill pad with 3-to-1 side slopes when the raise is small, and on piers and beams when it is larger or when Harris County's Class II rules apply. The stairs, ruler and labels show the heights.
The model is an illustration. Real pads, piers and grading need an engineer's design.
Before you build
- Ask the floodplain office for the 100-year and 500-year elevations for your parcel in writing.
- Hire a surveyor for an elevation certificate. A post-construction certificate is typically required.
- Ask about compensating storage if you plan to add fill.
- Flood insurance rates depend on how far the lowest floor sits above the BFE. Ask your agent.
- See our Houston Permit Checker for the permits that go with floodplain work.
This tool is a planning aid, not a floodplain determination or an elevation certificate. Rules change, so confirm every number with the floodplain office before you design or pour.
Sources
- City of Houston Chapter 19 Guidelines, summary of key provisions (August 2018)
- City of Houston floodplain FAQ on repairing flood damage (substantial damage, 50% rule)
- Freese and Nichols: new City of Houston floodplain regulations
- Regulations of Harris County for Floodplain Management, amendment draft of November 2017 (Section 4.05, Class II permit). The County adopted the amendment on December 5, 2017, effective January 1, 2018; confirm the final text with Harris County Engineering.
- Summary of Harris County's amended floodplain regulations (24 in above the 500-year elevation)
- Bisnow: how Harris County's new flood plain regulations impact construction (pier-and-beam in the 100-year floodplain)
- Harris County Engineering: Floodplain Management and permits
- 44 CFR 60.3 (NFIP floodplain management criteria, lowest floor at or above the BFE)
- FEMA Map Service Center and the Harris County Flood Education Mapping Tool
- HomeGuide: fill dirt delivery cost per yard
- HomeGuide: pier and beam foundation cost and house lifting cost
The Harris County reading comes from a draft of the amendment, not the adopted text, so it is the part to confirm first. The fill price range and the raise limit for fill are our assumptions.
Estimates only — not engineering, legal or code advice. Check requirements with your local building department and a licensed professional before you build.
Common questions
How high does a new house have to be in a Houston floodplain?
Under the City of Houston's Chapter 19 (effective September 1, 2018), a new home in the 100-year or 500-year floodplain must have its lowest floor at least 2 ft above the 500-year flood elevation. With this tool's defaults (500-year elevation 56.5 ft, existing grade 52.0 ft) that is 58.5 ft, which is 6.5 ft above grade and 6.0 ft above a normal slab that sits 6 in above grade.
Do additions and remodels have to be raised too?
In the City of Houston, a residential addition over 1/3 of the footprint, and any substantial improvement in the 100-year floodplain, must also be 2 ft above the 500-year elevation. A residential addition of 1/3 of the footprint or less in the 100-year floodplain must be 1 ft above the 100-year elevation, and has no requirement in the 500-year floodplain. A substantial improvement in the 500-year floodplain only has no elevation requirement. A substantial improvement costs 50% or more of the structure's market value.
What are the rules in unincorporated Harris County?
The Harris County floodplain regulations, amended with an effective date of January 1, 2018, require the top of the subfloor of the lowest habitable floor in the 100-year floodplain to be 24 in or more above the 500-year elevation, or 12 in above the crown of the nearest public street, whichever is higher. This tool applies that for new construction and substantial improvement. For additions and for lots only in the 500-year floodplain it says to check with Harris County Engineering, because the rule text we reviewed does not give a clear number.
Where do I find my base flood elevation?
Look up your address on the FEMA Flood Map Service Center to get your zone and, for AE zones, the base flood elevation (BFE). The Harris County Flood Education Mapping Tool (FEMT) shows the floodplains around your lot. The 500-year elevation is the less public number, so ask the City of Houston Floodplain Management Office or Harris County Engineering, or look at an elevation certificate for your lot.
Does raising the slab mean adding fill?
Sometimes. Up to a few feet, a fill pad or a stem wall under the slab is common. Higher than that, a pier-and-beam foundation is usually cheaper and lets floodwater flow under the house. In the City of Houston, fill placed below the 500-year elevation has to be offset with compensating storage. Harris County press coverage of its 2018 rules says new buildings in the 100-year floodplain need pier-and-beam foundations. Confirm with the floodplain office.
What does it cost to raise the floor?
Published ranges for foundation work are roughly $5 to $16 per sq ft for a stem wall and $9 to $12 per sq ft for pier-and-beam, and clean fill dirt runs about $5 to $30 per cubic yard delivered. For the default 2,000 sq ft house raised 6 ft, this tool shows about $21,000 for pier-and-beam (typical), with fill and stem wall shown as alternatives. Engineering, compensating storage and permits are extra.