Drzwi is a so called "plurale tantum", it has no singular form. Whether you actually mean one door, or many doors can only be derived from context.
Like in English, the words 'hair' and 'fish' are always plural and exact meaning being derived from context.
'In your house are narrow doors'. It is not necessary to say 'there'. Or, yes, one could say' There are narrow doors in your house' but when I reverse the wording I can miss out the 'there'. Oh...never mind...I just wanted a break. These exercises aren't enough to keep the mind alive.
I remember when I first learned (of course here, on Duolingo) that this 'there' is not obligatory... I was totally surprised. English never ceases to surprise me...
"Drzwi w twoim domu są wąskie", I guess. That's the closest natural sentence I can think of.
'są' is plural, drzwi means 'door' and 'doors', so I think you should accept 'There are narrow doors...'