So many variables - material choices, site access, roof shape, height, how it joins the existing, services etc etc, but it definitely won’t be £100k (£7000/sqm) and it’s quite unlikely to be £30k (£2000/sqm) these days unless there are lots of things on your side (experience, knowledge, contacts mainly). £50k ought to be more than enough unless it’s super complicated.
Here you go: https://costmodelling.com/building-costs
So many variables - material choices, site access, roof shape, height, how it joins the existing, services etc etc, but it definitely won’t be £100k (£7000/sqm) and it’s quite unlikely to be £30k (£2000/sqm) these days unless there are lots of things on your side (experience, knowledge, contacts mainly). £50k ought to be more than enough unless it’s super complicated.