My tip is get yourself an experienced independent agent/manager to supervise the build - or treat it as one semester of a college course on how to build houses,

My tip is get yourself an experienced independent agent/manager to supervise the build - or treat it as one semester of a college course on how to build houses, and accept the costs that you would expect (with the full realisation that you are going to have to do two more to "graduate"). When I first did projects I got caught repeatedly with items that were not on the spec sheets or costings - but were 100% required. This is one of the ways that builders manage price competition with the inexperienced. They 100% know what the rate that is likely to get quoted will be, if they want the work they will quote lower than that and will use overruns to make back to the market or somewhat above (because by then you are stuck). An experienced project manager will catch 95% of these issues some will sneak through but they will also have the contacts to get these sorted if the builders are unreasonable. You will pay out a chunk to employ this person - sure - but this will pay you back just with the cost overrun issue (in my experience) and at the same time you will have much less day to day work on your plate. So, it's basically a "free" assistant. Also, what's a good job? You have ideas... but experienced builders (and your manager) will know where it's really smart to spend your money.