The building length of the house is17.7m. Is the geomantic omen good?

By length, you mean depth, right? That's no problem. If the door is so wide, that's not good! Because long houses (similar to school classrooms) don't catch fire.

If you go deep, of course, the design of ventilation and lighting should be reasonable! This ancient mansion is very long and deep, but they will open the patio for lighting.

The most important thing about house feng shui is that it is suffocating, the hall is good, and there are no unfavorable mountain houses around (this will not be known until the scene). It would be a nice house with Long Mai in the back.