Not 100% clear on what exactly you mean, but BSP is a good way to prototype layout quickly. You'll end up needing static meshes if you want to decorate your level to a more high-poly standard. Some steps to help make decorating/building your level easier:
1. When figuring out what your level is going to be, visualize a couple of different spaces in your level. What functions do particular rooms have, what visual elements would be in a room with a particular function? Make sketches, or look at some of the available packages for UDK to see if there is a style or theme you want to go for. Having figured out functionality/look for your space is a good way to make it intuitive/easier to build the actual layout.
2. Build your level with some standardized sizes. Make it so most spaces have ceilings that are x meters tall, make sure that doorways are a certain width/height, so that you can easily generate more space without having to think much about the scale of things. Once you figure out a single room and a couple of basic elements like doors and windows, a layout becomes easier to build. Additionally, making floors/walls/ceilings/doorways/windows a defined width/height/depth and sticking to that will make it easy to set up a basic level of decoration later.
3. When you have built your layout, look for repeatable parts, like a 5 meter long wallpanel, or a 3 by 3 meter window, or a 2 by 3 meter door. Then find a mesh/texture package to get comfortable with, and mesh out single instances of what you want that doorway/window/floor/wall/ceiling to look. Once you've figured out one repeatable element, it should be easier to figure out the next if you want to create a consistent theme. When you've laid down the basic repeatable art, find spots where you can add in a unique feature/element to break up monotony.
4. Just build test rooms/spaces. They don't have to be full levels, but building small, contained spaces to the spec that you're thinking of helps make the process easier every next time you do it, even if the thing you're building right now isn't a portfolio piece.
