Plans, plans, and more plans.
Plumbers (Heating Contractors) bring in house plans, and I do material takeoffs from their square footages and other features. Basically, I figure out what sections of the floor will be heated with infloor hot water radiant pipe, and which areas are off limits or unheated. I then perform a heat loss calculation on the heated room areas, complete with window and door details (I hate it when architects omit window dimensions on the floor plans). I then often do infloor pipe layouts (yes, I draw all the pipe as it should be installed in the floor), and complete a package that the contractor can submit to the city to draw his permit.