(Please note that in certain situations,  what ,  when , and  where  can function as relative pronouns.) Relative pronouns introduce relative clauses, which are a type of dependent clause. Relative clauses modify a word, phrase, or idea in the main clause.
The 2014 Survey of Construction (SOC) from the Census Bureau shows that the average completion time of a single-family house is around 7 months, which usually includes around 25 days from authorization to start and another 6 months to finish the construction.
Building a custom home usually takes several months, or more, and unforeseen expenses squeeze your budget. Custom homes typically cost twice as much to build as existing homes in the same area, plus the cost of the land. The national average cost of building a two-story, 2,500 square-foot home is $125 per square foot. This estimate includes mid-range materials, efficient windows and doors, turnkey finishings and appliances.