We have started diging without submitting an Initial Notice! What should we do now?

I think I just replied to your last message....but first of all don't panic....For small scale building works you can usually submit a building notice application, which is sent a couple of days prior to starting on-site (I would usually recommend 2-3 weeks to be safe). You can then start the work, but do so at your own risk, since building control may have questions with the build, etc. You can also submit a full plans application, which isn't as common for house extension, which means you cannot start work until they are approved. You need to stop work and request for the inspector to visit. Leave the site as it is, you don't need to fill the holes back in. Then see what they need and take it from there. However I will mirror what i said in my last comment, which is if you are not sure what you are doing, then simply don't do it without first getting the right professional advice and a good builder onboard....