UK - Flat Roof Replacement

Looking at a property to buy and the seller has said that the flat roof needs replacing. The EPC rating also said it needs to be done.

It situated across the garage and kitchen of the house. It is approx 23m2 and it’s the one thing I’m struggling to price.

DIY - I assume materials would come at around £1.2k

Felt - I’m estimating £3k

Felt, Fascia and Plywood - £4.5k

Anybody have any experience/recommendations with this, or similar pricing from the past?