Well, big bros don't like lengthy chats on this forum. We are left with 2 options. My one - make it an answer, yours - accept it before reading further.
In short you have to modify DSM inside building outline.
- Use Geoprocessing - Environment _ Spatial Extent to set it to DSM.
- Use Geoprocessing - Environment _ Raster Analysis - Cell Size to set it same as DSM.
- Calculate average elevation for building, using it's outline
- Use Raster Calculator to raise a part of your elevation:
This will result in changing your of elevation from:
to
Repeat your viewshed analysis using what I call 'small' in above example.
When placing observers, I'd suggest to put them on concentric circles around the building. Expect to see bigger 'skyline pollution' for points closer to building in low places. Also avoid placing observers inside other buildings, because you'll have to deal with the issue of 'where the window are'. (I'd even erase buildings outlines from viewsheds).
On the other hand if this research is about animal welfare, e.g. 'California's Association of Stargazing Persians' you might very well place them on the roofs. Don't forget to place a couple on building of interest. These two will benefit from building upgrade and might request name changes later to Hillary & Tensing. TGIF