(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 Full Time (~40 Hours a Week) Those who can dedicate a full time effort to the project can usually get it done in about 2-3 months (usually a little longer than that) with experience and proper planning. It can, of course, and usually does- take longer (4-6 months).