Ultimately the amount you can borrow depends on your lender, but it also depends on what kind of self-build you’re working on.
Commercial projects
The first is whether you need the money for a commercial or a personal project.
If it’s a commercial project, which is going to bring in a return on investment, a lender can sometimes lend you up to 100% of the build cost, on the understanding that you’ll be making a profit on the finished project.
Personal projects
For a personal self-build project, you can borrow up to 95% of the total cost of your project from some specialist lenders.