In the US this could be done with a short term variable loan called a construction loan that releases money in stages as the build progresses. Once finished if it’s not being paid off it would be refinanced into a more traditional mortgage. Mortgages are often pretty different in the US vs UK, most US mortgages are for fixed rates for 30 year terms whereas most UK mortgages are fixed for a much shorter period and then go to variable rates. So you’d be hard pressed to get a bank to agree to a fixed rate 30 year mortgage for a house that doesn’t exist yet.
In the US this could be done with a short term variable loan called a construction loan that releases money in stages as the build progresses. Once finished if it’s not being paid off it would be refinanced into a more traditional mortgage. Mortgages are often pretty different in the US vs UK, most US mortgages are for fixed rates for 30 year terms whereas most UK mortgages are fixed for a much shorter period and then go to variable rates. So you’d be hard pressed to get a bank to agree to a fixed rate 30 year mortgage for a house that doesn’t exist yet.