Do you know how to do a residual valuation? This is the calculation a developer would do when working out how much to pay for a property, or how a chartered surveyor would work out the value if you asked them.
Start with the value of the house once the works have been done (you can work this out by looking at rightmove, or land registry to get sold prices)
Then subtract:
The cost of the works (get a builders quote)
A contingency (% of the building works)
Cost of financing the project - ie the interest payable on a loan whilst the works are being completed
Profit margin (% of finished price which you wish to pocket
Any other costs you can think of ie rent payable on alternative accommodation
The figure you're left with is the max that a developer would pay for a project knowing that they could still make a profit at the end of the day.
Assuming you're doing this for a family home, then you can decide whether to include the profit margin in your calculations and the finance costs.
And then you may wish to add on some extra money to help you secure the property if you're desperate to get it - only you can work this bit out.
I really hope this hasn't been teaching you how to suck eggs. Good luck!