I'm struggling to see what could a tiny 8sqm addition to already authorised extension possibly breach in the local plan? Are you envisaging a problem here?
Every council has their own planning policy, it's called the "Local Plan". Within it they provide policy and guidance with regards to how development should be considered within the borough and it's what the planners use to determine whether a planning approval should be approved or not. There will be a section devoted to domestic extensions ie house extensions, some local plans offer very little guidance on domestic extensions whereas some are quite restrictive eg you may only increase the area of a house by a certain percentage, so you should get your architect to check your councils local plan to ensure your planning application meets the guidance, otherwise you may struggle to get permission.
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