Sell rental property to fund new build?

Thanks Bronte, they are all valid points and we are trying to be clever with the design. I'm sure the years will fly by and it won't be long before it's just the two of us in the house. Any advice on the interior layout would of course be welcome
 
Hi Purple, no I haven't talked to the bank with which we have our existing mortgage, despite being aware of this option to move the tracker since they are not aware that we have let out the apartment and don't want to lose the tracker. Thanks for the advice