My experience is as follows... - A larger boiler is not necessarily needed (it wasn't with us) - I would agree that a 4 bed 2 bathroom house is more sellable than a 5 bed 1 bathroom house - Typical cost of the loft conversion is 30K. But after you've fitted out the bathroom, had the whole lot decorated and carpetted, paid for the building regs inspection, had fire doors fitted, electrical work, etc, 35-40K total spend is realistic. - It's a big job which takes a couple of months - It's a very messy job, so if you plan to get it done at any stage in the future, now is the time