Can I use an Orangery or does it need to be an extension... need help and advice on renovations

Hej, I do not think that your query can be answered as simply as a board posting, I think that you will need to employ and architect, however I am one so maybe I can shed a little light. Your problem is essentially your SAP calc or at least the minimum U Values for habitable space. You can make other design considerations under a weighted system such as using thermally insulated glass, full fill cavities and warm roofs in order to get you U values up so you can add more glass, this can be done and you can achieve greater spaces than the 25% of the new floor space you are looking to achieve. For the planning, most good architects will look at the local policy and guidance for your ward and be able to extract good information and therefore provide you with a substantive idea of what is likely to pass through planning, some of which will probably be covered under PDR, your rear extension for example qualifies for the Neighbour Consultation Scheme. The bottom line is that Conservatories and Orangery is not a habitable space, they are much more relaxed for the purpose of building control and cannot form part of a habitable space, however you can get your extension close by taking the correct design considerations. Tack Björn