Translation:In the apartment there is a bedroom, a bathroom and a kitchen.
Surprisingly, Duo rejected
"The flat has a bedroom, a bathroom, and a kitchen."
and insisted on "apartment" (correct in the US), though it accepts "flat", the commonest UK English word for 'mieszkanie' ("tenement" being used in Scotland) in many other sentences.
[Edited 09.08.2018 to include "tenement"]
This may come off as nitpicking, but let me explain what caught my attention. I'm a native speaker of Slovene, which has a very similar grammar to Polish. This sentence would have the exact same structure in Slovene, and most Slovenes would also use singular. It "sounds right", but it's actually grammatically incorrect because a verb in singular cannot refer to several things. I think people get confused if a verb in plural stands right next to a noun in singular, so they use singular instead. It's an extremely common error.
I suspect it's the same in Polish. This sentence might sound good to most Poles, but I'm wondering what someone with a degree in Polish would say about it.
I asked. I sent her the sentence we are discussing, and before I could even type my question, I got the answer "są". So yeah, you are right. I just wonder if it's worth changing the main answer, as changing the main Polish sentence is really problematic, and this one is so widely accepted...