"The chairs of the house"
Translation:Viti vya nyumbani
Nyumba means house. When you are saying 'at the house/in the house/of the house...etc' ,nyumba takes on the locative case which adds 'ni' to the end of it turning it into 'nyumbani'. Same thing for other places for example 'mkutano-meeting'. Ninaenda mkutanoni - im going to a meeting. See how it gained 'ni' at the end.
"The chairs of the house" is grammatically correct English, but I suspect you will (almost) never hear an native speaker of English use the phrase. I googled it and the meaning that came back was "wenyekiti wa baraza la wawakilishi" -- totally different meaning from what is intended here. If I saw the phrase "viti vya nyumbani" standing alone I would probably translate it as "house chairs", like "viti vya darasani" (classroom chairs), "viti vya ofisini" (office chairs), maybe "viti vya shule[ni]" (school chairs).