Check your sight lines, rights of way, restrictive covenants and potential need for new entrances.
If you can’t connect to the mains, you need to get a percolation test and pick an on-site treatment system.
Connection fees for these can run high, so check in advance although exact connection costs may not be forthcoming until you’ve got planning permission.
Check zoning laws, including locals only rules, and other site specific issues.
Consider that this can be an easy way to get planning permission; plus knocking down to rebuild.
Planning permission is the government’s written authorisation for you to build a house according to the plans you submit to them, oftentimes with conditions attached.