Level of upfront cash needed for a self build project

Hi - I am considering a self build project. I have found a plot of land and think I could get it for around £145k. The build cost would be roughly £200k taking overall project cost to c£345k. What I am trying to determine is how much cash I would need upfront in order to get the project rolling which I know is dependant on the terms of the self build mortgage but am just trying to get a rough idea about what I would need upfront.

Any advice appreciated.

Regards
Robson