Addition onto an aging home?

[Brian Fabel]

I own an old single family house built in 1890 in an area that the city zoning allows for duplexes and multi-family dwellings. The foundation is cobble and stone and in good condition given its age.

I was wondering if anyone had ever done a side-by-side addition to add another unit to an existing old single family? It doesn't seem that common, but I thought I would ask in case anyone had lots of experience in this area of blending the old with the new in pursuit of the best use of the land?