I'm going to install a central vacuum system and it would be easiest and most convenient to run the pipe out the side of the garage, run along the exterior side of the house, and then directly through the side of the house to service two bedrooms on the lowest floor.
That side of the house is a 6 foot alley between 2 houses, and the alley is completely sealed off and unused by people in either residence. So, there would be no aesthetic impact to the neighbor whatsoever. Indeed, both houses already have a lot of electrical conduit, network and coax cables mounted externally on the sides of the house in that alley.
This house is in Southern California, so condensation should not be an issue and in any event, the pipe could be wrapped if necessary.
More generally, what building codes govern? This is LA County, California.
Best Answer
Central vacuum pipe is thin-walled, so be sure to protect it from impact by kids, trash bins, etc. Direct sunlight will exacerbate this over time. Consider painting it to prevent UV damage.
Otherwise, I can think of no concerns aside from the aesthetic implications.