go online and find a radiator sizing chart, try stelrad for starters, then measure your own rads to see if you can work out the output, you need height, length and whether it is single or double panel, with or without fins in between, (look at the stelrad sheet then look down on top of your rad to see what you have) it wont be a perfect match but will be close enough for what you want, simply go round and write down all your rad size/designs then work our what your total output is at the moment, then you need to work out the requirement for the new rooms ( a wee bit trickier but post back on here and the good lads on here will work it out for you if you give us room dimensions) to use a REALLY rough guide multiply the room volume IN FEET multiply by 5 then divide by 3412 to give you kw output, once you have done all this add 2 kw for your hot water, this will get you close enough to see if you are above your 24kw