Here you can see how I was looking to solve that problem, the solutions to which I discussed the other day. As you can see the planned extension was also developed and in such a way as that I can still retain the roof section I've already constructed. Not only that, due to the 'plug and socket' module that the roof and extension use I could potentially keep creating more and more extensions as the the top and base are the same dimension!
My wife did a night shift the other night so I got in a lot of work and hopefully I'll be able to bring you up-to-date on all my progress next week. One post to cover the tower, the fences, the drop pod, dreadnought and even my AoBR marines!