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Weirdly, I don't think buyers would care that much about the space above garage compared to something funky about an aspect of the place that has little excuse to have something funky about it (and has been factored in to the sq. footage and floorpan).
There are some buyers who care about this stuff to the nth, some who are capable of taking a view. Probably correlates with whether they've gone for bargain basement solicitors.
You'll get the cash for it anyway, just not quite as much as if it was legit.