SE London extension cost

AnonMonkey

Hi all, 

I've just had an offer accepted on a house in South East London, or more specifically Welling. 

Can anyone tell me how much I should expect a 2.5m*9m (22.5sqm) single story side extension to cost near the area? Ideally this would create a single bedroom and a small bathroom. 

Additional questions if I may:
- How much will the cost increase be if I were to opt for a 2 story extension?
- Does anyone know if the single story side extension would fall under permitted development?

Thank you in advance!

AnonMonkey

If no one near the area has had an extension recently, can anyone in the south east area share their extension size and cost so I can get a rough feel for current extension costs?