View the property with a builder and get a proper quote. Don’t rely on a guess made by an estate agent who knows nothing about building work. If possible chat to the builder who has done the work so far.
Don’t rely on the plans meeting building regs. I know from bitter experience that it’s perfectly possible to get planning permission for something that building regs will never sign off! We got permission to replace a patio door with an ordinary window but building regs said we couldn’t replace an opening door with a non opening window because of fire regs. We had to rip out the brand new window and reinstate the fire access. I was like - if it isn’t permitted then why did they grant planning permission?! And they said it’s not the responsibility of the planning dept to check that your plans comply with building regs.
You do your sums carefully ( several times ) and know what you are in for financially before you start
You have the time , commitment , resources, money , vision and drive to want to do this type of project through to completion , the reality of projects like this is very different to sitting watching homes under the hammer from the comfort of your sofa
If you go ahead I wish you all the best , it's an amazing experience building and converting a house so exciting and so worthwhile