Will I have problems witht the textures if I do this?

I use blender for creating low poly objects and I am not sure what to do regarding the uvs

Let's say that I have a model of a simple house 4 walls and a roof, the wall are identical, can I unwrap the model and stack the uvs on top of each others in my 3d software (Blender) so they will take the same exact space in the uv island or this will cause me problem inside of unity3d when like backing lightmaps or any other thing?

 

Nope, you are fine. It generates its own uvs for light maps. Stacking uvs like you are describing is a common way to optimize texture space.

 

Great thanks.