It's "Safe as Houses". I've never heard of "Rough as Houses". Safe as Houses is referring to property being a solid investment over the medium to long term.
It's possibly Australian in origin, as they have similar phrases such as 'rough as a bag' and 'rough as guts' etc. A bag one can just about understand, if it was one of these jute/hessian coal-sack types, but the roughness of guts and houses are harder to comprehend. There is, of course, the word 'rough-house' meaning a bit of a barney, so there is a precedent for associating the two words.
Could you have misheard? I have often heard (and used) the expression 'rough as ar$eh0les', usually to explain how I felt the morning after. I suppose this could be corrupted to 'rough as ar$es'.
And now I am wondering why we said 'rough as ar$eholes'! I have used it so often and it doesn't make sense. Mine's not rough, it's as smooth as a baby's house.
I have certainly heard 'as rough as old houses' or 'as rough as old boots' which is generally taken to meaning something quite ugly or which has a face like ...
I have used this phrase often and always thought it was about the old council/tenement houses where us working class lived (outdoor toilets/bathrooms 'n' all).