The Foundation Library for
Factory-Built Homes
Plain-language guides to what the codes say, what lenders require, what things actually cost, and when you need an engineer — for manufactured homes, modular homes, Boxabl units, and everything else that arrives on a truck.
Start with your structure type
Manufactured & Mobile Homes
The four foundation families, the three rulebooks (3280, 3285, PFGMH), what FHA/VA/USDA lenders require, and when certification letters come into play.
Read the complete guide → IRC / IBCModular Homes
Built to the same codes as site-built houses, set on conventional foundations, real property from day one — and still full of lender questions worth answering precisely.
Read the complete guide → Folding / ExpandableBoxabl & Folding Units
What ESR-4725 covers and what it doesn't, the four foundation families for Casita-class units, dual-module bearing, and the permit path through your AHJ.
Read the complete guide →Common foundation questions
Each article starts with a direct answer, then works through the details — and names the source documents so you can check for yourself.
What is a permanent foundation?
The PFGMH's five criteria, in plain language — and why screw-in anchors don't count.
Foundation certification letters
Who asks for them, what they say, and how to get one in 1–2 days.
Foundation types, compared
Pier, crawl space, slab, basement — with the honest tradeoffs.
Boxabl Casita foundation requirements
What the install manual and ESR-4725 actually specify — and what stays site-specific.
Modular vs. manufactured
The classification that changes your mortgage — and how to tell in five minutes.
Foundation cost ranges
Honest ranges by type, and the five site factors that move the number most.
Tiny homes & park models
Do they need permanent foundations? The financing reality, category by category.
Anchoring & tie-downs
What inspectors look for — and the one component installations most often miss.
When you're ready for the engineering
This site is for learning. When you actually need a letter, a plan, or design work, those services are handled separately — by the same licensed engineers at Oasis Engineering.
Manufactured Home Certification Letters
PE-sealed PFGMH foundation certifications for FHA, VA, and USDA loans. Flat $450, 1–2 business days.
Start a letter →
Boxabl & Modular Letters + Plans
Foundation letters and site-specific plan support for Casita-class and modular installations.
Start a Boxabl letter →
Container & Full-Service Engineering
Container home structural plans and complex or custom foundation engineering, direct from the firm.
Explore container homes →