Can't tell you why but this building gives me a constant "Object reference not set" error. It's a shame too, seeing as it's a really nice building!

It should definately be possible to have a lower tris count, but it can be hard to tell how much (or how little) detail to keep. I'd be happy to take a look at your LOD and see if I can give some pointers to get it lower but still keep the quality of the model. You can also peak at how others have handled similar shapes and see how much detail they kept. Kliekie's high school comes to mind as a good example of an irregular shape and a low tris LOD. :)

But I have made the size of textures for the main model small. Frankly speaking I thought that 100 triangles is too little for display high-quality lod-model

Thanks for council. :)
Unfortunately I didn't manage to make less quantity of triangles with such geometry.

This looks very nice! Though I gotta say 700+ tris for a LOD model is too much - remember the goal is to get as low as possible and the recommended is 100 for buildings. It's a shame it's so high, since the building looks really nice. :/