1. Dear *****, I regret to say that if a right of way is only for agricultural and equestrian purposes, you cannot subsequently use it for residential purposes. It makes no difference if an alternative right of way which you have been using, gets revoked on you. There is no such thing as expanding a right of way through necessity. You either have it or you don't.
2. Accordingly, there is no such thing as expanding this agricultural and equestrian right of way to a residential right of way in the future, just because you get stuck with having another right of way revoked or withdrawn on you. I would instead advise you to negotiate with whoever is the owner of the land which, this right of way, or the other right of way runs over.
3. Essentially, you should nail down the right of way issue before you go building the house. Either expand the use of this agricultural and equestrian right of way to include residential use or get the other right of way made permanent. But don't leave yourself in the lurch by building and having a right of way withdrawn.
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