If building and heating your extension means moving a boiler then you need to be aware that under building regulations, this is classed as a new installation. So it's not just as simple as moving your current boiler - if the boiler you want to move is not a High Efficiency boiler you will need a new one for your extension to conform to building regulations.
It's definitely worth raising these issues with your builder or project manager in the planning stage of your extension, as you might have to factor in changes to your heating system in your overall budget and timeframe.
From a central heating perspective, when moving a boiler, the main thing to consider is if your current boiler has the capacity to cope with heating your extension. The more rooms it has to heat, the longer it will take to reach your desired temperature. Remember also, that with a larger house there will be more heat loss, so your boiler will have to work harder to compensate.
