There are a couple of main ways to approach radiant floor heat layout. The basic question is wether or not electricity or hot water is going to be providing your heat.
Electric floor heat is very much more flexible in terms of layout, and does not need so much forethought. For instance it is possible to install the electric systems simply by pulling up the carpet. In some cases pads are simply stuck to the bottom of non-fitted carpets and rugs.
By contrast retrofitting hydronic radiant floor heat will probably entail wrenching up the flooring itself.
Hot water radiant floor heat is however much more cost effective in most settings will happily replace any other form of heat. Electrical underfloor sometimes needs supplementing.
So water systems clearly need radiant floor heat boilers - the first layout question is where to locate these items. In many cases the boiler can do dual service - gas powered combination boilers like this are very common.
The underfloor layout is fairly straightforward - hot water pipes traverse the floor beneath the boards or tiles. This is why fitting it as the house is built is the most efficient way. But is often worth the upheaval for older properties, given the advantages of efficient underfloor heating.