kitHaus is another new modern modular housing provider. They're awaiting approval from the ICC for some of their larger structures but you can get one of the really small ones since it qualifies as an accessory structure.

Quoted: Welcome to the official kitHAUS website. A true modular site constructed pre-fabricated housing system.

Weehouse makes some very nice looking pre-fab modernist housing.

Ikea has begun selling prefab houses in Sweden and is rolling them out in the UK.

Quoted: BoKlok homes don't exactly come in flatpacks, but they're not far off. The timber-framed buildings are almost entirely prefabricated. They are usually brought to the site on the back of trucks as pre-assembled units, like Portakabins, with the interiors already fitted out. Each apartment is made up of two of these units, which are simply moved into position by crane. Put on the roof and exterior wall cladding, plumb and wire it in, and it's ready to live in. The typical BoKlok arrangement is an L-shaped, two-storey block with three apartments on each floor. One such block can be put up in a day...