I may be wrong but as far as I am aware each room in a house/apartment should be measured in terms of area to heat (Cu. metres) and then the size and number of radiators is decided according to their rating etc. I would imagine there are guidelines for this laid out in the builing regs.Check out the building regs. If it's a new apartment then I would enquire with the builder or your solicitor may help. To add an extra rad now would probably be costly and, as most new apartments are basically concrete boxes, it could turn out to be unsightly if the piping can't be covered.