Just because a builder is providing fixed-price doesn't mean the client is free from unscrupulous behaviour. Many fixed-price builders are well-versed in assigning changes and up-selling the client - indeed some base their business model on it!
Getting a good fixed-price for an under-specified home is no better than a high estimate but for a better specification - as the project progresses the client will typically come to realise the gaps in the specifications and need to make changes, unfortunately that can be prohibitively expensive. Get it right at the outset i.e ensure you have a detailed design and materials specification.
Work with a design team that intimately understands build costs so that they can balance form and function to match a pre-defined budget. The fixed price contract in that instance will be for the finished home and the margins charged are always more favourable than for changes made mid-project.
If you engage an architect/designer be clear with them what your budget is, otherwise you will get a home that looks great but may not be buildable within your constraints; it is surpising the mistakes that can be made by experienced designers when they don't visit the site or understand the local conditions or costs.
© Brecan Buildings Supplies Ltd, 13 Kutenai Road Fernie, BC, V0B1M5