Almost every project that goes wrong goes wrong before production starts. This is the list, in the order the answers are needed.
The plot, in writing
What the local plan permits on that specific parcel, for the use you actually intend, for the duration you actually intend. Ask the authority in writing and keep the reply. A verbal yes at a counter is not a decision and does not bind anyone.
If the use you want is not permitted, that is the whole project — not a detail to work around later. Settle it before anything else is ordered.
The utilities quotation
Distance to water, power and drainage, priced. Utilities charge by the metre and quote lead times that in busy regions run to a year. This number frequently exceeds every other site cost combined, and it is free to obtain.
Get it as soon as the plot is identified. It decides the foundation, the energy strategy and sometimes the plot itself.
Access for a lorry and a crane
Entrance width, turning radius, height under branches and cables, and what the ground will carry. Photograph the approach, the last bend and the final position, with a tape in shot for scale, and send them before the order is confirmed.
A difficult access is solvable. A difficult access discovered on delivery day is a second crane visit and a wasted crew.
The specification that cannot change later
Structure, insulation build-up, service routes, window positions and the loft. Everything on that list is fixed at production and expensive to alter afterwards; everything not on it — finishes, loose furniture, appliances — can wait.
Spend the decision effort there, and let the cushion colours wait until the house is standing.
The money, including the parts nobody quotes
Unit, transport, customs and VAT, foundation, connections, crane, planning fees, insurance, and a contingency. Then the annual figure: maintenance, filters, the yearly inspection, and for a let unit, cleaning and replacement.
Model the letting case at ten points below your expected occupancy. If it does not carry the fixed costs there, the project depends on a good year.