So it was Hattori!!! I was right!!! Apparently, Hattori had some business with the detective agency and the kids asked for his helped to be able to defeat Conan. But unfortunately, they still fail. Too bad for them :(
So the secret of the house is on the ceiling. The window that you can see outside is just a fake and the real window is above it. The ceiling can be moved up and down, giving an extra space upwards. The way to move the ceilings is through the abacus on the wall. Conan was able to figure out on how to manipulate the abacus based on the dust the accumulated.
So it turned out that the treasure hidden in the house were all stolen items from the recent robbery case. Some treasures from the Bakumatsu era were also there. Also, inside the house was the robber himself, hiding from being seen by people.
What a very interesting case this is!