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Depends what is fucked but getting ours taken back to the rafters and redone was ~£8k in 2021. Around the same size, 5 x 4m and at height.
Flat roofs are comparatively simple - basically a load of boarding on joists with a water proof coating on top and sealing around the edges - basic lead work.
They might have quoted for a whizzy new coating like EDPM or sheet metal or a ‘warm roof’ for better insulation?
£20k? What are they doing, cladding it in gold?
Guess a load of structural stuff has Blown?