Put in the big numbers first
Add a likely section price, build cost, the value of your current home, your mortgage, savings, household income, and an interest rate. The numbers already in the calculator are examples only. They are not a guide to the market.
Remember the other costs
Add site works, services, council costs, legal fees, builder changes, somewhere to live, and a buffer for surprises. It is better to spot a missing cost now than after you sign.