Hi Stuart
It sounds like you needed building regs on the first design as it's attached to the house and it being a sizeable development so yes you would need building regs on the new design so new designs showing the structure and depths of footing and all construction, insulation etc
If it's not within permitted development then you will need planning permission as well but I am sure you've had advice from whoever is drawing up the design
You can do quite a lot now on existing houses without planning permission but you will need building regs I am sure because the structure is being attached to the house
