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It varies and depends on their hourly rates but you can say that this is a direct instruction (not via a law firm) and as such can you have "end of case payment terms", or a part payment now and balance at conclusion. If it's a small claim (£10K and under) then they would only have to wait for up to 6 months for the payment (worst case scenario).
I would think anything between £500 to £1000 would be usual which should include travel time and preparation of a report which complies with Part 35 of the Civil Procedure Rules 1998 (and therefore to be admissible as evidence).
The fee is recoverable from the builder if you win the claim (I should have added).