Hi, thanks. Well, first and foremost I am surprised that your Solicitor did not raise this issue while you were purchasing the property. I can't commit and say that they have been negligent, but anything built over a right of way is a major issue as it could mean the owner faces injunction proceedings and the ultimate possibility that the building/extension has to be removed. This is because it is very very hard to prove that a right of way has been "abandoned".
An easement, (right of way) can be sometimes impliedly released by the owner’s actions or in rare cases by the owner’s inaction. It can on rare occasions be established that a right has been abandoned. However, this is a not easy to establish as at law there is no obligation on a party to exercise that right. Failure to do so will not automatically result in an easement or right of way being released due to the assumption that it has been abandoned. If the owner explains the non use he or she may still be regarded as not having abandoned the right. Failing to use an easement or right of way is not of itself sufficient and abandonment will not be inferred. The owner must make it clear that he or she is abandoning the right not just for himself but also for his successors in title.
In the case of Benn v Hardinge (1992) 60 P&CR 246 the Court of Appeal said that the failure to use the right for 175 years was not enough on its own to indicate an intention to abandon.
There is an assumption that the right has been abandoned where it can be shown that the original character of the land has been changed to such an extent that the right of way has become unnecessary or impossible to exercise. (As the road has never been built, this argument would be your best defence).
However, this is only a presumption and can be rebutted by the owner producing evidence to show that the original character of the land can be restored at a later date and that the need for the right would be revived (ie that the road may get laid and the garages ultimately used).
This is a complex area of the law that requires a scrutiny of the title deeds and actions or non action of the party claiming a right of way or easement and the actions of his/her predecessors in title. I would therefore suggest that you take independent legal advice if this matter is not resolved to your liking by the letter your current Solicitor is going to send. I am sorry this may not be the answer you were looking for, but it sets out the general law. If I have assisted, I would be grateful if you could rate my answer. Kind Regards Al