Not dodging it, a simple way of showing compliance is to use the U values stated in a table, however you can adopt other approaches ie average U value calcs, this allws you to compensate for reduced insulation in one element by increasing other elements, but for such a small extension I would make a judgement, for example increasing the insulation in the roof of the proposed work, topping up the roof of the existing house or having insulation blown in the wall of the existing walls. Its about reducing CO2 emissions.
What you have to appreciate the approved documents are a method of showing compliance, they are not regulations.