How many metres can I go out from main building for rear kitchen extension?

Permitted development will only allow you to extend to 3m projection. The neighbourhood consultation process will allow you to informally ask your local council if it's ok to build out up to 6m. There should be no fee but I have heard of some councils charging an admin fee for this and you will need to submit a basic sketch showing the proposed extension. There may also be a basic form to fill out too. They will then write to your immediate neighbours informing of your proposal and then give you a yes or no decision. This will take up to 42 days and if you don't hear anything after this time you are then entitled to build. If you don't do all this then you will have to stick to the 3m allowance (better to be safe than sorry). Louise UK Property Services

You can go as far as you want as long as you get planning permission.

There are set guidelines and parameters that you need to abide by if you were looking to extend as part of a permitted development, however these do not apply if you are intending to apply for actual planning permission. In theory you can apply to extend to the sizes you have listed and beyond. In reality the local planners will consider if your proposals are in keeping with the area, oversized for your plot and if there are any objections etc etc. A good pointer is if there are any other similar sized extensions on neighbouring properties, if there are then you should have no problems as these would set precedence but if in doubt have a pre-application discussion with the planning officer. Regards David