Trying to learn for near-term application, already got an initial quote from a relative but want to fully understand the process and what we can/can't do, what needs to be fixed/decided immediately and what can wait and think a bit more.

Current property already some extension (ground floor) done, second floor we want to extend as well. Will need some extra footings done (these aren't cheap I understand) to accomplish this. Ground floor also will need to hack some walls (we want a much more open living area, and our family size is fixed hence number of rooms less of a priority.

The plan after extension is some built-ins (particularly in the kitchen and three bedrooms, plus upstairs bathrooms). In general, countertop, large sinks, island, wardrobes. Haven't decided on fixed dining table, but probably we want flexibility to move that.

This will mean a fair amount of re-tiling (potentially just about the whole house) with accompanying painting. Before which we will need to drop some plug points, probably some ethernet for smart home/internet use, add A/C points for rooms which don't have, etc.

So for all the work being done, what order does it have to be done in? Is it better to have a single contractor from start to finish, or have a contractor for structural works and another for finishings? Ideally we have the complete plan right at the start with no changes, but looking at our decision making process, its possible that we may want to make minor (non structural) tweaks, for e.g. partition board for a store room, mobile island instead of fixed island in the kitchen. How much leeway do we have to make those changes from the time structural work starts (contractor estimates 3-6 months depending on weather)?

If there are advise/FAQ type threads/resources to read up I'd be very interested as well.