Know the number before you commit to it
Self builds rarely fail because of one bad decision. They fail because a plot was bought on an optimistic figure, a design team was appointed before anyone knew what the ground was like, and by the time the real cost appeared there was too much already spent to walk away. A feasibility study puts that number in front of you at the only stage where you can still act on it freely. Land, construction, professional fees, finance, contingency and the abnormals specific to your site, assessed properly and set against what you can actually spend.