Factory-Built & Modular Unit
Foundation Certifications
Including Boxabl Casita installations

A sealed engineering letter for your factory-built unit on its permanent foundation — for the lender who needs to see affixture, or the building department that needs to see conformance. You take the photos, our app guides every shot, and a licensed Professional Engineer performs the review and seals the letter.

1. 1Start your letter below — property, unit and contact details, plus a few questions about how it was set (3 minutes).
2. 2Guided photo session — about 30–45 minutes at the property, any phone. The app tracks your checklist and confirms when your photos are complete, so nothing gets missed.
3. 3Licensed engineer review — a PE evaluates your photos against the approved plan set. If any shot needs another try, the app reopens just that item with exact instructions — your progress stays saved.
4. 4Sealed letter delivered — by email with a durable download link, typically 1–2 business days after your photos are submitted.

Modular is not manufactured — and that difference is worth money

A state-approved modular unit is built to the International Building Code and International Residential Code, inspected under a state industrialized-building program, and set on a permanent foundation designed for the site. It is not a HUD-code manufactured home: no HUD label, no 24 CFR Part 3280, and HUD’s Permanent Foundations Guide for Manufactured Housing does not govern it.

We only write that sentence when the photographs actually support it. If your placard is missing or unreadable, the letter says what we saw and no more — and we will tell you before you pay for something that won’t help.

Meet the engineer behind your letter

One minute from Enrique Lairet, PE — the licensed engineer who personally reviews and seals every letter.

Oasis Engineering is the firm behind EngineerLetters.co. These reviews are about our foundation certification work generally — the same licensed PE reviews and seals every letter, whatever the unit.

“This is the smoothest experience we’ve had with an engineering certificate — anybody at our company. We’re definitely going to be giving you more business as it comes through.”

Matthew Martinez · loan officer, National Mortgage Home Loans · by phone, Aug 2026

Start your certification letter

For a unit that is already set on its foundation.

Need a foundation plan first?

Factory-built units ship with a flat base. The foundation underneath is a separate design, permitted separately, and the manufacturer’s drawings say so in as many words — the concrete is always “designed and built by others.” That is engineering we do.

Tell us about your site and a licensed engineer will reply personally — not a form letter, and not a sales call. If the answer is “you don’t need us for this,” we’ll say that too.

What the letter covers

The letter does three things: it identifies the unit from its build data placard and approval-agency insignia and records whether a HUD label was present; it states that the as-built foundation, the unit-to-foundation anchorage, and the observable connections conform to the approved plan set and the manufacturer’s installation manual; and it gives an engineering opinion on permanency and affixture. Where a lender asks for it, HUD’s PFGMH is cited as a recognized benchmark for permanency — explicitly by analogy, never as the governing standard for a modular unit.

It is based on photo documentation captured under our guided protocol plus the approved plan set, and it says so plainly. Concealed construction — reinforcement, embedment, footing dimensions below grade, concrete strength — was not exposed and the letter states that too. The engineer retains the unconditional right to request retakes or a site visit, and no certification is issued until the evidence supports it. Read our full methodology →