Next decide the area it is going to be located, if relevant, and search out suitable ground/property. If you can negotiate an option on the property, i.e. its yours for an agreed price if you commit within an agreed time limit, often this will not cost you anything up front, but may make getting the price down more difficult.

Take great care of access arrangements, particularly when you are dealing with land. The land may appear to join the road but you may find a small strip inches or feet wide, owned by the council or someone else which costs you a third of the total value of your land to get hold of. Some people specialize in buying up these invisible strips for just this reason.

Consider risks. Flooding, land subsidence, access for fire engines and hydrant pressure, where the nearest sewer works or other sites are that could be annoying on a hot day when the wind is in the wrong direction.  If its a site for children, are there other dangers, rail line nearby, ponds or lakes. What about noise?

Set about identifying problems, planning permission, listed building consent, fire rules. Identify architects, stock design drawings etc  and get quotes from builders or help from a self build organization in costing up the project.  Watch out for council arrangements in some towns where you have to buy non existent car parking spaces to go with each unit you create. Talk to local developers, estate agents, builders, to identify what you have overlooked.