A fixed price or lump sum contract gives cost certainty and is usually the right choice for a house build, but it only works if the design and specification are complete before pricing. Incomplete information turns a fixed price into a variations exercise.
In a lump sum contract, the builder prices the drawings and specification and carries the risk of getting the quantities wrong. That risk is priced in, so the tender may look higher than a cost-plus quote — but it will usually be closer to the final figure.
Cost-plus, where you pay actual costs plus a percentage, suits renovation work where the scope cannot be fully defined until walls are opened up. It requires trust and open-book accounting.