You must be prepared to be flexible when you encounter something unexpected. Flexibility is having items that are not sacred cows and which can be sacrificed should the need occur such as going for cheaper floor coverings or fixtures and fittings should the need arise.
It is not uncommon to pass self-build homes where the owners have needed to delay the final landscaping and external finishing due to cost overruns during the build. Some self-builders will install second-hand kitchens and paint their concrete floors until money become available to finish things off.
If you intend to apply for a mortgage, then it is best to speak to your bank or building society at an early stage and understand their requirements. The situation is different in NI and ROI. Be mindful that some banks will not lend if the house does not conform to their standards, e.g. they are wary about non-traditional roofs such as large flat roof areas, and so are insurance companies, or sites with a shared road access, for example. The test question comes down to how easily can the bank sell the property if the self-builder is no longer able to pay off the mortgage?