The small house in the middle of the lake is rustic. The walls, while stone, are roughly hewn and the roof seems to be put together from random logs and rushes. There are two windows, one on either side of the door, and the glass is mostly intact. The door is slightly open and is wooden. The hinges are rusted, but do move. Once inside, though, things change.
The building is only an anchor point for what ever "magical house/refuge/extra dimensional space" your campaign has. The house is actually quite large inside. There is a large kitchen and pantry area, a dinning room, three bedrooms and a study inside along with a greeting room which has a nice fire place. (There is a fire place in the kitchen, as well.) Everything is perfectly preserved from books to furniture. It looks as if the owner just left.
Treasure in here consists of normal household goods, a small magical library, (your players can find d12 +5 new spells and notes on what ever you want to throw in), and a small stash of gems and jewelry. There is magically preserved food, lanterns and oil, and some camping supplies in a small storage room.
This was the retreat for a mage who needed time away from his family to study. He died several centuries back, and his heirs had no idea this small home was in the tunnels. He has maps of all over Martapa, (or your campaign world), though some are a little out of date. The biggest change is the fact that a demon scholar has taken up residence in here and is a large reason no one makes it out of the tunnels alive. Make this demon appropriate to the party level and ability.
His treasure will be book treasure, plus a little extra from all the adventurers he's had to kill that have disrupted his studies.