Non traditional housing – one of the main styles that were used was concrete houses.  They used what was called large panel construction (large pieces of concrete bolted together to formt he house walls).  Used wimpeys no fines which means concrete without any fines in the cement.  Fines are things like small stones, the result was it was quicker to build that way.  This sort of house is generally considered ok to have a mortgage on whilst the others are non.