From what I've observed in the 1851 census, Plymouth and the Three Towns were mostly numbered by that date (though renumbering may have taken place subsequently). I suppose it depends where you are, though I would have thought that if a house number existed it would have been quoted in Kelly's directory. If you can find the family in context within the 1851 census, and you have a street map to go with it, you can sometimes guess at a house location by working out the route the enumerator took.