Extension cost is driven by floor area, structure, glazing and — more than most people expect — the level of finish. Answer six questions for an estimated build range.
How much floor area are you adding?
Total new internal floor area
Single or double storey?
What level of finish do you want?
Kitchens, flooring, joinery and glazing
Will it contain a kitchen or bathroom?
Are internal walls being removed or large openings formed?