It depends what you mean by private road, or unkept road. AIUI a road can be 'unsurfaced' in that its not tarmac, but it might still be a type of construction such as some hardcore, gravel, wood chipping, etc etc I'm not sure if that's what you mean by unkept, or whether you mean private as in privately-owned.
A road can be unsurfaced yet still maintained (and paid for) by the local council. Or it can be an unadopted road, which means its not maintained (paid for) by the council.
The ownership of the land the road is on is another issue - a public road can cross private land, and that land underneath the road surface is still 'owned' by the landowner. If its a right of way, then the council has certain responsibilities and the landowner has certain responsibilities.
And then there's right of way - which is the right to drive over the road etc (or for footpaths, bridleways, walk/ride horse/ride cycle/drive animals etc as appropriate).
If there's no public right of way then your property and others may have a right to use the road via the deeds, or informal permission of the landowner.
Best to seek proper legal advice on the status of the particular road you're interested in and the process to get a proper access agreement in place. Doing it yourself may upset the landowner(s), or result in more restrictive than you wanted agreement, or too high a cost for it, etc etc - or none at all, which would severely devalue the prospective build!