Budget and financing
Deciding on a budget is very individual. It could be what money you have saved or the value of your current home, or even the maximum amount a mortgage provider will lend. This is definitely not the time to be over optimistic – if anything, underestimate what you can afford. At the very least come up with a conservative amount that you are prepared to spend. And that is all you need to get started.
Remember that the budget is a moving beast as changes occur and specification decisions are made; also be under no illusion about knowing precisely how much something will cost before it is designed, specified and sourced.