There's a sink. Does that mean I can install a toilet?

[waternwavs]

It's a ground-floor flat with a cellar underneath. Is it likely I would be able to see the relevant plumbing if I ask the agent to show me the cellar?

One other detail: all the other parts that used to be offices are now flats. As the communal toilet in the stairwell is so covered with dust that it clearly isn't used on a daily basis, would you say that is an indication that installing toilets to this remaining flat is physically possible?

Incidentally, the flat is the yellow part of this diagram. The sink is in the wall in the smaller room called 'Lokal nr 4' on the right, at the top (i.e. the wall opposite the window) and 'wspólne WC' is the communal toilet.

https://www.sspma.org/uploads/8/3/9/2/8392851/oberkorn_sspma_wwett_2018.pdf

"4. Sewage Pump: A pump powered by an electric motor for the transfer or removal of wastewater from a sealed basin to a sewage treatment device or collection system. Solids-handling sizes of Sewage Pumps are typically 1” through 2” diameter. "

Just one idea below..