FEMA’s Elevation Certificate uses eleven standardized building diagrams, numbered 1A through 9, to classify a structure by its foundation type and how the space below its main floor relates to the surrounding ground. The diagram you enter in Section A, Item A7 controls where the surveyor places the measuring rod and which elevation fields get filled in, so it is the single most consequential entry on the form. Pick the wrong one and every measurement downstream feeds bad data into your flood risk profile and your community’s compliance records.
Why the Diagram Drives Everything Else
Once the diagram number is recorded in A7, it tells the surveyor which of the eight elevation fields in Section C (C2.a through C2.h) apply and exactly where on the structure each reading is taken. A slab-on-grade home calls for a measurement at the top of the concrete slab. An elevated coastal home on pilings may require a reading at the bottom of the lowest horizontal structural member instead. The diagram also signals whether the surveyor needs to document enclosures, garages, or crawlspace areas below the building.
The diagram does a second job too. It communicates the building’s physical characteristics to insurance underwriters and floodplain managers. Whether a lower area is open to floodwater flow, partially enclosed, or a full basement changes how regulators evaluate compliance and how insurers assess damage potential.
The Eleven Diagrams by Foundation Type
Each diagram represents a distinct foundation configuration. The descriptions below reflect the official FEMA classifications from the Elevation Certificate instructions and the NFIP Flood Insurance Manual’s Lowest Floor Guide.
Slab-on-Grade: Diagrams 1A and 1B
Diagram 1A covers standard slab-on-grade buildings where the bottom floor sits at or above ground level on at least one side. It applies to single-floor homes, multi-story buildings, townhouses, and high-rises, with or without an attached garage, and it’s the most common diagram for homes built directly on a concrete pad.
Diagram 1B is for raised slab-on-grade or slab-on-stem-wall-with-fill construction, where the slab has been intentionally elevated above the surrounding ground using a stem wall and fill material. Same building types, but with that raised foundation detail. The distinction between the two is whether the slab rests at natural grade or has been lifted above it, and that difference changes where the surveyor takes the bottom-floor elevation reading.
Basements: Diagrams 2A and 2B
Diagram 2A covers buildings with a basement floor below ground level on all sides, across homes, multi-story structures, townhouses, and high-rises. Diagram 2B also covers below-grade basements on all sides but applies when the internal floor layout differs from 2A. In both cases, the surveyor measures the top of the basement floor as the lowest floor elevation.
Basements produce some of the highest flood insurance costs because the lowest interior space sits below the surrounding ground. If the basement floor is below the base flood elevation, expect a substantially higher premium.
Split-Levels: Diagrams 3 and 4
Diagram 3 is for split-level buildings on slab-on-grade construction, including townhouses, with or without an attached garage. Diagram 4 is for all other split-levels, such as those with a partial basement or a subgrade crawlspace.
Split-levels are singled out because they have multiple floor elevations that don’t align neatly with a single “lowest floor” reading. The diagram tells the surveyor which of those levels is the controlling one.
Elevated Buildings: Diagrams 5, 6, and 7
Diagram 5 is for buildings raised on piers, posts, piles, columns, or parallel shear walls with no obstructions below the elevated floor. The area underneath must be completely open to floodwater flow, though open lattice or insect screening is allowed.
Diagram 6 uses the same elevated foundation types as Diagram 5 but covers buildings with a full or partial enclosure below the elevated floor. These enclosures typically must have flood openings that let water pass through. FEMA requires at least two openings on at least two walls, providing a minimum of one square inch of net open area for every square foot of enclosed space.
Diagram 7 covers buildings elevated on full-story foundation walls with a partially or fully enclosed area below the elevated floor, including walkout levels where at least one side is at or above grade. The main living space must be in the elevated portion. A raised ranch with a walkout basement generally fits here, because the lower level has full-height foundation walls with one side at grade and the primary living area is upstairs.
The difference between Diagram 5 and Diagram 6 matters. An open foundation lets floodwater pass underneath without building pressure against walls, which generally translates to lower risk. An enclosed area must meet flood opening requirements to avoid trapping water and increasing structural damage.
Crawlspaces: Diagrams 8 and 9
Diagram 8 is for buildings elevated on a crawlspace where the crawlspace floor is at or above grade on at least one side, with or without an attached garage. Diagram 9 is for buildings on a subgrade crawlspace (below ground level on all sides), excluding split-levels, again with or without an attached garage.
A Diagram 9 crawlspace sits entirely below grade, putting it in a similar risk category as a basement. A Diagram 8 crawlspace has at least one side at or above grade, which typically means less flood exposure and a different measurement approach.
How to Pick the Right One
The FEMA instructions direct the certifier to “select the Building Diagram that best represents the building” and enter that number in A7. If no diagram is a perfect match, choose the one that most closely resembles the structure.
Two questions get you there in most cases:
- What is the foundation type: slab, basement, split-level, elevated on posts or piles, sitting on foundation walls, or over a crawlspace?
- Is any area below the main floor open, enclosed, at grade, or below grade on all sides?
The certificate also requires photographic documentation. Section A, Item A6 calls for at least two, and ideally four, clear photographs of the building, one from each side. These photos help verify that the selected diagram actually matches the structure and give reviewers a visual cross-check against the recorded measurements.
Which Measurements Each Diagram Triggers
Once the diagram is selected, the surveyor records up to eight elevation readings in Section C, Item C2. Each field captures a specific point:
- C2.a is the top of the bottom floor. This is the most commonly referenced measurement and the primary indicator of whether the building meets the base flood elevation requirement.
- C2.b is the top of the next higher floor, relevant for split-levels and structures where multiple floor levels need documentation.
- C2.c is the bottom of the lowest horizontal structural member, required for buildings on elevated foundations (Diagrams 5 and 6 especially) and the controlling measurement in coastal V zones.
- C2.d is the top of the attached garage slab, since garages often sit lower than the main living floor.
- C2.e is the lowest machinery or equipment servicing the building, such as an HVAC unit, water heater, or heat pump.
- C2.f is the lowest adjacent grade, the lowest ground elevation touching the foundation.
- C2.g is the highest adjacent grade, the highest ground elevation touching the foundation.
- C2.h is the lowest adjacent grade at the lowest elevation of a deck or stairs.
Not every field applies to every diagram. A simple slab-on-grade home under Diagram 1A doesn’t need the C2.c reading. An elevated coastal home under Diagram 5 absolutely does. The diagram drives which fields the surveyor fills in and which get marked not applicable.
The comparison that ultimately matters is between the lowest floor elevation and the base flood elevation, the water height expected during a flood with a one-percent chance of occurring in any given year. When the lowest floor sits above the BFE, the risk profile improves. When it sits below, premiums climb fast.
Coastal V Zones Change the Measurement
In V zones (coastal high-hazard areas), areas seaward of the Limit of Moderate Wave Action line, and other areas regulated for coastal flooding, the measurement rules shift. Instead of measuring at the top of the finished floor surface, the surveyor measures at the bottom of the lowest horizontal structural member, typically a beam, joist, or girder supporting the elevated floor.
Coastal floods involve wave action, and waves slam into the underside of structural members. The relevant question isn’t where the floor surface is; it’s how high the lowest piece of structure sits above the surge. Buildings in V zones almost always fall under Diagram 5 or 6, and the C2.c field becomes the controlling elevation for both compliance and insurance rating.
When a Change to the Building Forces a New Diagram
A completed Elevation Certificate does not expire on its own. It stays valid unless a physical change to the building invalidates the certified information. A new flood map alone does not trigger a new certificate if the building hasn’t changed.
A new certificate is needed when:
- The structure has been improved or repaired to the point that the work equals or exceeds 50 percent of the building’s market value.
- Additions, enclosures, grade modifications, or other alterations would change any answers recorded in Sections A, C, E, or H of the existing certificate.
Routine work that doesn’t change elevation, foundation type, or enclosure characteristics won’t require a new certificate. But enclosing a previously open area beneath an elevated building, converting a Diagram 5 situation into a Diagram 6, absolutely does. It changes both the diagram selection and the building’s flood risk characteristics, and both belong on a fresh certificate.