The mention of the title deeds was useful as it encouraged me to look into it. From what I can gather, I don't think the title plan or deeds will have to be changed because of the new build, as we are not altering any boundaries. So we can probably build the main house and move in, demolish the current property and build new triple garage with granny flat in its place, and then inform mortgage lender (or not, as the mortgage is for the same address for a house that will be worth much more). If we can keep the relatively small existing mortgage whilst the build is completed then this would be a great help to us. If I inform them of our intentions before any work is started, then I am sure we shall have to re-mortgage in some way or other, and we are not in a good position to do this.