Modelling best practice - can I build a library of meshes? I'm new to blender and I've been trying to model a fairly simple house as a starting project. I'm thinking of building the various parts of the house as separate models, e.g. creating a model of a four pane window and then saving that as a reusable window (for any future projects), so if I want to create a house in the future, I can just load up the window and add it the model of the house. After some time I could have a large library of reusable objects. Is this considered good/bad modeling practice? should I be extruding/loop cutting a single mesh instead?
Yes, a library is a great practice. You can then link or append those library files into a current project. I recommend using append because you can then edit that particular object in you current project file.

https://www.blender.org/manual/data_system/linked_libraries.html

I also build a library of materials.