No, on balance, I think you're probably right about a tiny little gap that you can only access by sending a cat down it!
But if there's such a desire for a house that people can think of as 'essentially detached - not really a semi', I don't see how a three-foot gap couldn't be factored in when planning the space.
It's already a real waste of the available land, adding a single-storey garage/storage room on to the side. If space is at a premium but buyers are clamouring after the idea of detached/kind of detached houses, the obvious solution is to alter the layout of the house slightly so that the garage is integral, with a bedroom or two on top of it.
That way, you can make the house as a whole a bit narrower, whilst still having more liveable room inside, thus freeing up a bit of space for a narrow pathway between the properties and making them actually detached.
In fact, considering that so very few people now actually use garages for keeping cars, I'm surprised that more developers don't offer houses with an extra reception room (or indeed just plenteous internal storage space) instead.
Maybe there's an ingrained idea that a house without a garage is somehow lacking and inferior, but if you're only going to convert it and/or use it for storage anyway, surely it's better when it's already been done for you?
Garage doors aren't particularly attractive in the first place, so if you don't need/want vehicular access, what's the point?