The word is not German and seems to have been invited, and trademarked, by Countryside properties. They say "An überhaus™ is a home that, whilst set in an apartment building, is so much more than just an apartment. Each home is designed to explore the use of space: additional bedrooms, split levels, generous outside space and private ground floor entrances...". Stripping out all the sales jargon it sounds to me like just a larger than average flat or maisonette.
Phil Conran ● 4538d