Unfilled planning conditions

It's not legally unsaleable, but its value is signficantly affected. A purchaser will want to get a new planning permission and complete the development in accordance with that permission and all its conditions, and have completion and satisfaction of conditions properly certified. This may mean undoing and redoing some of what has already been done, either because different planning conditions are imposed, or simply because the work already done wasn't certified at the time and can't now be certified.

Exactly how big of a hit to market value you are looking at depends, obviously, on what work has already been done, whether and to what extent it was compliant with the permission and was certified as compliant, whether any new permission will be in similar terms to the old permission, and how much rectification and completion work will be required. But as the vendor I wouldn't be expecting to turn a profit on this transaction.

One also needs to check if the house is properly located on the site as per the pp, I would start with that, especially with vision line issues