Derrived from laying a brick were one takes a crap however it is where someone goes for an exceptionaly long trip to the toilet. those about to "build a house" may be spotted entering the toilet with some form of reading materal.
(conversation on msn)
mike: brb mate, gotta lay a brick
(45 minutes latter)
paul: holy hell that wasnt laying a brick - u just "built a house"!