It should be noted that the euphemistic usage of "outhouse" for a place to take a dump accessible only via the big room appears to be an exclusively American one, and, to judge from Webster 1913, of more recent date that that. In those areas of the English-speaking world where faeces-related euphemism remains one step behind the world leaders in the subject, such that one does not crap in the bathroom (that being a room with a bath in it; other facilities optional), "outhouse" is simply a synonym for the generic "outbuilding". Figuratively, very sturdily built people in such areas are from time to time described as being "built like a brick shithouse". The terms "outside toilet" or "privy" will generally be used in in the UK or Ireland; "dunny" is also known, especially in the Antipodes. Note that this is not (or, at any road, has within living memory not been) an exclusively rural phenomenon; the privy was once a common feature of terraced houses in the industrial areas of the UK until relatively recently; they were not always removed when the luxury of inside plumbing arrived and can still be found in a secondary role in a few houses.

Oh yeah, and an ad hoc installation that you create yourself while doing the Great Outdoors stuff (or on military field service) would be a latrine, while the temporary installations for public events are mostly known eponymically in the UK as Portaloos, the trade name of the market leader.