'Filmed' at Casa Loma Castle in Toronto
The irony of how cheap and stagey this movie looks is delicious considering it was 'filmed' inside a real castle - Casa Loma in Toronto (X-Men and a bunch of other movies have shot there too). And at Pinewood Studios Toronto.
https://www.thestar.com/entertainment/2016/10/16/on-set-in-toronto-with-the-cast-of-rocky-horror.html
The 1975 RHPS was filmed at Oakley Court mansion outside of London, but the ballroom and lab were shot at the adjacent Bray Studios where many of the Hammer Horror films were shot.
This exemplifies the power of shooting on 35mm film with good lighting vs digital cameras and lousy soap opera lighting. The 1975 version was shot by Peter Suschitzky who also shot Empire Strikes Back.