Will I have to sell my current house before my new home is ready?

This will depend on how you wish to finance your build and how much cash you have in savings. If you choose an arrears stage payment mortgage, then you will need sufficient money for a deposit of between 15% and 25% of the land cost as well as money to pay for the materials and labour for the early stages of the build. If you do not have sufficient savings, then you may decide to sell your current house to release equity before starting your new project
Receiving your stage payments in advance during the project will ensure that cashflow is not an issue during the build so you can continue living in your current house until you are ready to move.