mildew smell in brand new house?

Sounds like you have water issue between floor and mesh under the floor. When they build modular homes they put them together and check the plumbing for leaks. If it leaks they fix it, however, most places do not replace the insulation after they fix the leak, figuring it will just dry out on its own. BUT, that is not always the case. Sometimes the water sits in the insulation and causes pests such as mice and rats to come in and build nest, causing even further headaches. Whether or not the pest arrive or not it can still cause a black mold problem in the insulation.

In other cases, the modular may have been set up on the lot with water attached, then sometimes the toilet may get used and left to sit until it is ripe, causing a smell that stays for quite some time.

New building?

If this is a new building with new car, flooring, and other finishes and maybe even some new furniture; it is likely from the chemicals leaching from the materials.

Take Tony's advice and air the place out.

Also make sure your traps are filled with water in the washroom and anywhere else you have any type of drain.

the windows have been close for a long time , so ventilation is important , check the bathroom area first and work on that

Is it on a basement? If so put a dehumidifier down there. If a crawl space, did you have vapor barrio installed? If not, it is probably coming from the crawl space.

air the place out