The most common budgeting mistake is starting with a single figure and working backwards. A better method is to work forwards: list what the project must include, price it in sections, and then see what the total says.
Start with the scope, room by room
Write down every room and what happens in it: flooring, wall finish, ceiling, electrical points, joinery, furniture, upholstery, glass work. A kitchen and a bedroom with the same floor area can differ widely in cost, because the joinery and services content is so different.
Separate the big categories
Keep civil and flooring work, woodwork and polish, glass, upholstery and electrical as separate lines. This makes it easy to see where the money goes and where it can be adjusted without damaging the design. Materials and labour should also be visible separately wherever possible.
Allow for what is not yet known
Concealed conditions, a change of mind about a finish, or an added piece of storage will all come up. Keep a contingency line, and agree in writing that any deviation is communicated before it is bought or built.
Remember the extras
Professional fees, transport and cartage, and GST are often left out of early figures and then arrive as a surprise. Ask for all of these to be shown clearly so you can compare quotes on the same basis.
A budget built this way is longer to prepare, but it is far easier to defend and to adjust once work begins.