Hi,
Extensions are pricey especially if you go further Down south...much money is often spent on moving drains, foundations and steels. If you feel the quotes you are receiving are over priced, these are a few things you could do consider:
1- If anyone in your area has completed a similar conversion, ask them what it roughly cost them...this way you know you're on the right track.
2- Ask your builder for a complete break down from foundation work, brickwork, and roofing to electrics, plumbing, plastering, joinery and decorating- a complete breakdown. This will allow you to see where the costs are coming from and allow you to prioritise things over others. Our builder gave a breakdown to the T including prices for knocking down existing walls, installing steels.
3- You say your builders refusing work unless you agree on the whole lot... it may be that the particular builder is a contractor and only takes on whole projects. We found it easier to use a contractor as opposed to finding different tradesmen individually...For us it helped the renovation work flow.
4- Your architect or designer may be able to five you a rough guideline price based on the area.
5- does the work your having done involve moving drains, installing drains, significant pipework or electrics, reboarding, plastering,etc. These little things add up.
6- Get multiple quotes, ask local builders as opposed to larger companies- they are often cheaper...You could possibly take on some labouring to minimise costs, if it's your thing, offcourse.
Good Luck with everything...hope it all works out.
Q