"Craftsmen built houses"
Translation:Mafundi walizijenga nyumba
"Craftsmen built houses" contains no hint that an object infix might be needed. So "Mafundi walijenga nyumba" should be accepted. How to give more of a hint that an object infix is wanted is a good question. Maybe "These are the very houses that the craftsmen built" -- Hizi ndizo nyumba ambazo mafundi walizijenga.