Modular (or prefabricated) construction is the same as site-built
construction, only the work is done in a factory, usually some distance from
the jobsite. It must meet the exact same criteria as any sitebuilt and is
inspected by independent third party inspectors (usually required to be
certified or licensed in the state the project is shipping to.) Quality is
usually quite high, and most often complies equal to or better than local
You can usually order an addition to your exact specifications, however plan
on still having considerable work to be done on-site (the foundation,
opening existing exterior walls, tying in the new structure to the existing,
etc. But as most of the addition has already been built, the time of on-site
installation is low, usually a couple of days or so (compared to a couple of
weeks for all site-built).
As quality will vary between manufacturers, best to check carefully as to
what others say each particular manufacturer. Often the price will vary
widely between dealers, so this is one area where a good reputation is
needed. (Does he use licensed personnel such as electricians, plumbers or
just hire whoever walks in off-the-street)?
All things considered, you could save considerable money by going this
route, but I would suggest you also get a couple of site-built bids for
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