Underfloor heating and cooling is a form of central heating and cooling which achieves indoor climate control fo thermal comfort using conduction, radiation and convention. The terms radiant heating and radiant cooling are commonly used to describe this approach because radiation is responsible for a significant portion of the resulting thermal comfort but this usage is technically correct only when radiation composes more than 50% of the heat exchange between the floor and the rest of the space. Underfloor heating runs at a lower temperature than radiators, so your builder's suggestion to run the underfloor heating all the time, controlled by the thermostat makes sense. You use your thermostat to set the temperature you want. It is an efficient way of running the system.
Underfloor heating is normally efficient because it effectively turns the whole floor into a radiator. The large surface area means it doesn't have to be a high temperature to warm the room – only a couple of degrees warmer than normal room temperature. For your underfloor heating to be most cost- and energy-efficient, your property needs to be adequately insulated. To prevent heat loss, and to ensure that the heat is directed upwards, there needs to be room for insulation to be installed beneath the system.