Double height extension on 1920s House - possible issues?

Chickereeeee

Hi All

I have a 1920s built, L-shaped detached house. Double fronted, bay windows, stained glass etc. All glass is double-glazed or has secondary glazing. Cavity wall, no insulation, near the coast. I am considering having a two-floor extension built, which would make the house a 'square'. Internally, this would add another bedroom/en suite upstairs and make the kitchen into a large kitchen/diner/garden room. The extension would be about 5m x 5m. I have been trying to think of any issues that would need addressing, and would welcome any comments. I am thinking of such things as:

1) Houses from the 1920s don't have deep foundations, but a new extension would.

2) I would want the extension to have the best insulation. Heat pumps and other green measures in the future need to have good insulation to be effective, but the rest of the house would not be as well insulated.

3) I would probably replace and extend the roof over the extension, and would like easy access for storage, although I don't need another living space.

4) The roof (half existing house, half extension) would be south facing, and so would be suitable for solar panels.

Any thoughts on specific issues to do with the above, or other, welcomed!

Also, who would it be best for me to start talking with, to get some initial ideas going? Builder, architect, specialist extensions company?

(In the back of my mind is the thought that, to meet future government ''Green'  measures a future owner might just demolish and rebuild to new government standard!)

Thanks

C