i know its cheeky but maybe knock on the door and ask the owners if they converted the loft and which building company they used? This is the most direct route. You could also contact loft specialist companies and request a ballpark quote based on the plans you have posted. The cost will also depend on the specification, the findings from a site inspection and various other factors. You said you have your eye on this property. Have you viewed it or made an offer? If you can show the buyer that you are seriously considering making an offer they may allow you to bring a builder to a second viewing.

Did you ever go ahead with this? Any idea what the costs would be?

What price did you end up getting for this at the time? We had a loft conversion done ourselves and I really wish I’d had something like this beforehand. I used this for all my other renovation projects to get a free ballpark estimate based on location

https://renocost.co.uk/loft-conversion-cost/loft-conversion-cost-uk/⁠