Could be any number of foundations. Here we go again with the definition of modular vs manufactured housing.
As a general rule, and only a general rule...it should contain concrete support piers and the support beams should be secured (anchored) to the piers. A perimeter concrete (block is acceptable) wall is also the preferred method. That said, I have seen engineered systems that are metal tripods with only skirting that will pass muster.
For that reason, there is no real good general answer to your question. If in doubt, call for an engineer's report!
Yes here we go again. Manufactured homes are constructed to the national HUD building code--look for the little red label at the rear of each section. For FHA financing an engineer's inspection of the foundation will be required. For conventional whatever the local government will provide. And like Mike said make it subject to an engineer's inspection regardless if you are not willing to accept the local government's inspection.
Modular homes are built to the same building code as site built homes, UBC, BOAC, CABO, etc, etc. Therefore since it is a local building code that has to be followed for the construction of the home, the local building code must be followed for the installation, foundation, perimeter enclosures, attached structures, etc.
So make sure you are dealing with a modular home first (DO NOT depend on appearance, materials, craftsmanship, etc, etc, etc to make the decision)--then find out what the local government for the subject site requires. Make it subject to verification of approval by the local government entity. And if you are in a state where modulars are inspected and approved by a state agency that applies an insignia or label for modular homes--look for that, take photos, write down in your notes everything that is on that item.
Just out of curiosity, how many modular homes are sold in your area versus HUD coded manufactured homes. I would bet it is less than 5% in TX. Now, they call them all modlular, but usually they are HUD coded manufactured homes.
Decided to jump back in regarding foundations for manufactured homes constructed in the factory to HUD building code. The best resource to have on any appraiser's desk, next to Fannie Mae guidelines and most current copy of USPAP, just in case they ever get an order for an appraisal for a manufactured home is the "Permanent Foundations Guide for Manufactured Housing", costs $25 from HUD. It has illustrations of the various foundations accepted by HUD, except for units placed over a basement. Then local building codes have to be followed.
Basically there are three types, Type C: Foundation system supported and anchored at chassis only, to equally spaced piers. Type E: Foundation system supported at chassis and exterior wall but anchored for uplift and overturning at exterior wall only. Type I: Foundation system supported at chassis and exterior wall but anchored for uplift and and overturning at exterior piers only. The exterior foundation wall (AKA as the perimeter enclosure) are called non-bearing walls or skirt walls ith they are not used structurally. Foundations must be constructed of durable materials: concrete, mortared masonry, or treated wood and be site built. One of the Type E foundation systems that is acceptable is a treated wood perimeter wall on concrete footing with unanchored metal piers (pages A-27 & A-28).
One of the neat things about buying the book, it has the illustrations of acceptable foundations. But it does take an engineer to inspect and approve for FHA financing.
Didn't see your posting Tim until after I posted. The number of newer modular homes in Arizona are very few. There are still a few (very, very few) old Wingfoot by Good year that were built during World War II, and few people even realize what they are--but they have been updated with indoor baths! During the 1970s there were two builders, Ray Homes and Bullock Homes, that built a few modulars that are scattered across the state. Few people know they are modulars because there wasn't any type of identification, the building departments and the assessor's office treated them as site built. So unless the original owner or a neighbor remembers the home being trucked in 30 years ago, nobody knows it is a modular. Then about 15 years ago Cavco at one plant in Phoenix started building on frame modular homes to UBC on steel undercarriages, used the same floor plan and materials as their HUD code homes, were towed to the site, wheels axles tongue removed and set up just like a manufactured home. They cannot be identified by appearance as a modular home--have to find the modular home insignia. But they circumvented zoning laws that prohibited HUD code homes. There are probably less than 1,000 in the entire state. The manufactured homes to HUD code are probably in the 100s of thousands in the state. About two years ago Redman Homes under the name of Genesis started constructing off frame modular homes on wood floor beams which are trucked to the state. They look exactly like all the site built stucco frame tile roofed homes in all the new subdivisions in Phoenix and Tucson. The only way to identify them will be that they will have a crawl space instead of a concrete slab. Their insignia will be hidden in the attic. There are probably less than a 1,000 of those so far. They are building their manufactured homes to HUD code using the same floor plan, design, material, etc as their modular homes. The modular homes will have 9' side walls and high ceilings since their roof unfolds. The manufactured homes will have 8' side walls and a vaulted ceiling. Their website is http://www.genesishomes.com Look for the Hamilton Homes at the Chandler, AZ plant, I think they have a plant in Texas also. The Genesis modulars will be treated by building, zoning departments and assessor's offices just like a site built home.
For both types, your best bet is the manufacturer (Not the dealer). They typically have a staff engineer that can help you.
Some, not all, manufactures are supported by the steel beams that run down the center of the unit, the perimeter wall is NOT load bearing. The perimeter "foundation" nothing more than a skirt regardless of whether it's made of concrete or plastic. Thats why when they are installed on a basement there are I beams that span across the foundation for the homes I beams to rest on.
If you are really lucky there are local building code requirements that will take the monkey off your back.
Concrete isn't always required, I've seen some engineered steel foundations that were required in some areas in California because they were superior to concrete with respect to earthquake resistance.
Modulars (UBC, CABO, code) are typically supported like a stick built home on the perimeter.