Medieval houses generally made out of timber were often being raided by several dragons that can breathe chemical fire at a distance. They seldom touch the houses with their razor sharp talons nor topple it with powerful tail, they simply flew around in circles and spew fires(napalm) at the houses and trees. What kind of houses can the medieval folks built to stand up against these airborne flamethrowers?
When all else fails, dirt is reasonably cheap and fire resistant. Build the house out of a frame, and then just slather a thick layer of mud all over the top and sides of the house which should provide a reasonable protection against fire. Also serves as a decent insulator too.
The frame of the house can be wood, but roofs, with proper reinforcement, can be slate. And the walls can be daub and wattle. The wooden wattles wouldn’t provide protection, but the daub would be fire proof since it is little more than gypsum and plaster, treated with water proofing.
Also, if needed, green hides of slaughtered animal could be nailed to walls and doors to provide additional protection.
Give the dragons a different target.
Use fake houses - more just shaped piles of kindling that turn into a bonfire when set alight. Even use strawmen as well, to make it look habited. The real houses are all low profile, hidden underground wherever possible, but the decoy houses are used to draw the dragons fire elsewhere, to a safer part of the city.
The dragons come every week, burn down the decoys, but it's easy to just rebuilt afterwards.
Otherwise, what do the dragons actually want? If they never land, then presumably they're not actually raiding. If they just want to snatch meat, then pay them off - leave out fresh butchered meat for the dragons, and presumably leave it in the decoy houses for the dragons to take. Dragons get fed and leave, nobody gets hurt.
If you want to be spiteful, poison that meat.
It's best to build your houses from either dirt (as mentioned by other answers) clay, brick and mortar, slate, or stone blocks. Any stone or aggregate based building material would stand up to fire much better than organic materials like wood and thatch. That being said, a bunch of napalm-like fire would still do significant damage, so damage control would be something you'd look into.
This would be my choice if dragon attacks are on the order of the day. In medieval times (and even now), there's hardly a better defence imaginable than a massive slab of mountain or dirt overhead. The only thing that can get damaged is the entrance, and if you have multiple this isn't too much of an issue.
A bit of an out-there solution, but not infeasible. The romans managed to build massive aqueducts to irrigate their fields and cities. A large aqueduct network throughout the city could help massively with firefighting. Built of stone bricks and doused in water, the aqueducts themselves would stand up to the fire attacks quite well. Any houses that would be hit and burning, one could remove the shutters on that side of the aqueduct and douse the house in water, instantly putting out the fires. This could even be done pre-emptively when a dragon is spotted, so that the wet houses would not take as much damage.