Reference

Glossary of Foundation Terms

By Enrique Lairet, PE · Oasis Engineering LLC Reviewed August 17, 2026

Plain-language definitions for the terms used across this library, written for homeowners first with enough precision for lenders, appraisers, and builders. Definitions follow 24 CFR 3285.5, the PFGMH, and ASCE 7 usage; where industry slang differs from the regulatory term, we say so.

ADU (accessory dwelling unit): A secondary dwelling on a residential lot — backyard cottage, guest house, or a delivered unit like a folding-box home. Many jurisdictions have streamlined ADU approval paths that small factory-built units can fit.
AHJ (authority having jurisdiction): The local body — usually the building department — with legal power to review plans, issue permits, and inspect. The AHJ’s interpretation controls, even where manufacturer documents exist.
Anchorage: The components and connections that resist uplift, sliding, and overturning. “Rated anchorage” means the parts have been designed and tested for specific load values — the crux of the permanent-foundation definition.
Auger anchor (ground anchor): A steel shaft with a helix plate screwed into the soil; the standard manufactured-home wind anchor. Excluded, by itself, from the PFGMH’s permanent-anchorage definition.
Base flood elevation (BFE): The elevation of the 1%-annual-chance flood on the local flood map. In flood hazard areas, the home’s lowest floor must sit at or above it.
Bearing capacity: How much load the soil can safely support, in pounds per square foot (psf). Drives footing and pad sizing; 1,500 psf is the regulatory default when the soil type is unknown.
Chassis: The permanent steel frame a manufactured home is built on and transported with. It stays with the home for life — one of the defining features of the HUD-code product.
Chattel loan: Personal-property financing, like a vehicle loan — the default for manufactured homes not affixed to owned land. Typically higher rates and shorter terms than a mortgage.
CMU (concrete masonry unit): The hollow-core concrete block used for piers and walls; nominal 8×8×16 inches. Stacking rules for pier height are set by 24 CFR 3285.303.
Crawl space: The enclosed, non-habitable space under a home on a perimeter-wall foundation, typically 18 inches to 3 feet of clearance, with ventilation and a ground vapor barrier.
Data plate: The paper label inside a manufactured home listing its design wind zone, roof load zone, and thermal zone. Found in a utility closet, near the electrical panel, or in a bedroom closet.
Exposure category (B/C/D): An ASCE 7 classification of the site’s wind surroundings: B sheltered suburban, C open terrain, D flat water-adjacent. The same structure carries different wind loads in different exposures.
FHA / VA / USDA: The federal loan programs whose rules make foundation certification a routine requirement for manufactured homes. FHA’s permanent-foundation standard is the PFGMH.
Finished floor elevation (FFE): The target height of the completed floor relative to grade. Drives drainage, flood compliance, utility stub heights, and stairs — lock it early.
Footing: The element that transmits loads to the soil: a pad under a pier or a continuous strip under a wall. Reinforced concrete, below frost depth, for permanent-foundation purposes.
Frost depth (frost line): How deep the ground freezes locally — from zero in the South to 100+ inches in the far North. Footings bear below it to avoid heave; the AHJ sets the number.
FPSF (frost-protected shallow foundation): A design using perimeter insulation instead of deep footings to prevent frost heave, per SEI/ASCE 32. An accepted alternative in many jurisdictions.
Geotechnical report: A soils investigation establishing bearing capacity, soil type, and groundwater conditions. Often required by the AHJ; always decisive when it exists.
Grade beam: A reinforced concrete beam at or near grade providing continuous support lines, often bearing on improved soil or deeper elements.
HUD code: The federal Manufactured Home Construction and Safety Standards, 24 CFR Part 3280. Preempts local building codes for the home itself; effective June 15, 1976.
HUD label: The red metal certification label on the exterior of each transportable section of a manufactured home. Its presence is the fastest classification test there is.
ICC-ES / ESR: ICC Evaluation Service and its Evaluation Service Reports, which confirm a product meets code requirements — e.g., ESR-4725 for Boxabl’s structural insulated panels. An ESR certifies the product, not your site or permit.
Longitudinal ties: Straps, struts, or bracing resisting forces parallel to the chassis I-beams. The most commonly missing anchoring component and a leading cause of failed foundation reviews.
Marriage line: The junction where sections of a multi-section home join. Requires its own piers or supports — and for dual-module folding units, unified bearing behavior.
Manufactured home: A dwelling built on a permanent chassis to the HUD code (post-1976), transportable in one or more sections. Distinct from modular (state/local code) and mobile (pre-1976) homes.
Modular home: A factory-built home constructed to the same state/local codes as site-built housing, set on a conventional foundation, and treated as real property from the start.
Park model RV: A recreational vehicle built to ANSI A119.5, capped at 400 square feet, titled as an RV — which is what usually keeps it outside mortgage financing regardless of its foundation.
PE (Professional Engineer) / seal: A licensed engineer legally responsible for a design or certification; the seal (wet or digital) is the formal act of taking that responsibility. Many AHJs and all major loan programs rely on it.
Permanent foundation: Under the FHA/PFGMH definition: durable materials, site-built, rated anchorage, reinforced concrete footings below frost depth, and a continuous perimeter enclosure. Screw-in anchors alone don’t qualify.
PFGMH: HUD’s Permanent Foundations Guide for Manufactured Housing (HUD-4930.3G, 1996) — the standard for permanent-foundation compliance in FHA/VA/USDA lending on HUD-code homes.
Pier: The support element between footing and home: stacked CMU, steel jack stands, treated wood, or listed manufactured piers. Height rules come from 24 CFR 3285.303.
SDC (seismic design category): An A–F classification assigned to a site from location, soils, and risk category — never an inherent property of a building product.
SIP (structural insulated panel): A factory-laminated sandwich panel — facings bonded to a foam core — carrying structure and insulation in one element. The Boxabl panel pairs steel and MgO facings over an EPS core.
Skirting: Non-structural weather enclosure between the home and grade. Not a perimeter enclosure wall, and not a substitute for one in permanent-foundation reviews.
Stabilizer plate: A metal plate at a ground anchor’s head that bites the soil and keeps the anchor from rotating under load.
Stem wall: A short perimeter concrete or masonry wall rising from a footing, creating a crawl space and elevation control between ground and structure.
Tie-down / strap: The steel band connecting chassis (diagonal) or sidewall (vertical) to ground anchors. Vertical sidewall straps are required in Wind Zones II and III.
Title elimination / affixture: The state legal process that retires a manufactured home’s vehicle-style title and merges the home into the land’s deed — the paperwork half of becoming real property.
Vapor barrier: Polyethylene sheeting (6-mil minimum under homes; also under slabs) blocking ground moisture. Required in enclosed under-floor spaces except in arid regions.
Wind zone: HUD’s three-zone map setting anchorage requirements: Zone I interior, Zones II and III coastal/hurricane-prone. A home may not be sited in a higher zone than its data plate shows.