Scale

Ok, So remember we said these guys are 6 inches tall?  That's shorter than your average Beer bottle.  Think about houses. Common Ceiling height averages around 8ft to 10ft.  So 8-10 inches.   Add a couple inches for the roof and you may breach 1 foot. Keep in mind if the farmer and his wife need water, the trolls need to be sure their house won't get destroyed or inspected for looking like a dollhouse that some deranged kid set up under a bridge.  It should be able to be easily overlooked.  Is it hidden?  Is it in plain sight but just looks like a left over beaver dam?  Etc.

Also, because its smaller, they probably won't have your average wood plank construction unless they are meticulous OCD trolls...which norse tales refer to them as somewhat messy, and range in cleverness.  (I see the wife as more put together and the husband of the more violent dimwitted variety).  Window frames, door jams, etc can still be there but will they be made of a split stick instead of a plank?  Made of pebbles?

If you were only about a hand tall how would you construct your house?  Most people can only lift 1/3 -3/4 their weight.  Trolls you can play with, but it needs to be believable.  Think of Barron's projects.

Sorry if this is last minute, but I think it will help with some confusion and style issues.