You can use existing radiators in the house calculations will be made to determine which radiators need up sizing accordingly. Where possible up-sizing can be achieved by moving the larger existing radiators into the smaller rooms and fitting new bigger radiators into the larger rooms. This reduces waste and overall cost of installation.
Underfloor heating is an effective way to heat a new build but not always the best solution for a retrofit. In many cases, radiators can be sized to deliver similar efficiencies to underfloor heating.