Is my boiler big enough?

I have a 4 bed house 1,400 sq ft and am about to extend by an additional 1,000 sq ft. I will be adding an extra 2 bedrooms and a large open plan kitchen/living room. I cant find any markings on my 15 year old current boiler in relation to its power consumption or energy rating (not sure what the units are). I was wandering if I will need to replace this by a new boiler considering the extension. Possibly a condensor boiler?
Any one any idea what size/power boiler I would need for a 6 bed house approx 2,400 sq ft.
thanks.....

You'll have to take some guidance from your plumber on that.

Every 2400 sq ft house is different regarding window ratio, insulation levels, orientation, hot water cylinder, underfloor or rads etc etc.

To do some back of fag box calcs see here, make a note of each room [broken link removed] add 10% and then see here for system sizes
[broken link removed]

Note the radcalcs can be a bit on the high side.

Calculations apart - the ratings on boilers often refer to their maximum output - changing the jet on the burner adjusts to the required output and minimises boiler cycling. Anything other than a condenser is irresponsible - preferably a good quality high efficiency model - the rest of europe has standards for this - we do not. Consider a variable speed circulation pump if you have thermostatic valves.