You need an architect because, technically, the works you are proposing are structural ie taking down and rebuilding, even though the walls will be exactly where they were before, and therefore you require a major works licence. For a major works licence you need an architect.
You may be told by agents, builders, neighbours etc that you can do these works under a minor works licence or whatever other licence exists in your area and you may decide to risk it and you may get away with it. But you may not.
Ask yourself, if you don’t employ an architect, who will be responsible for the design of your extensions? Would you build an extension in the UK without using an architect or similar?