How much roughly would a simple verandah/conservatory cost with dwarf walls a couple fo windows a door and a lean to corrugated roof be. Add an outside loo with a new basin and some heating and move the drains and maybe do some new foundations? I think our quote of �20-25,000 (London) is far too high.
Most builders do not do detailed costing to estimate your extension but work out the major costs of the structure by using the cost/per square floor footage. This can be in the range of �100/square foot upwards. On top of this would go the added costs such as bathroom or kitchen fitments or any non standard additions. Most builders are registered for VAT so add another 17.5% on top. You may find a jobbing builder who does not charge VAT but they are rare.
Ask around friends and nieghbours to get a personal recommendation for a builder also I would think you would have to get planning permission so your architect or surveyor
would be able to give a quote .
You give clue about size, but if the conservatory was about 15 square metres in floor area then �20 to �25k is perfectly realistic. Especially in London