The money to be loaned is split into stages and released at the beginning of each stage. The first part will be released to pay for the land and the rest is released as the build progresses with no money held back until completion. The usual stages are:

Purchase of land
Preliminary costs and foundations
Wall plate level
Wind and watertight
First fix and plastering
Second fix to completion

Arranging your self build mortgage in this way allows you to have a positive cash flow throughout the build. It also means that you have the funds available for the land purchase and to buy labour and materials as needed. It can mean that you do not have to sell your home until the build is completed, provided you can fund the payments into both mortgages.