you need planning permission which may involve visits from a building control officer to start with.
And yes you should excavate to the correct foundation depth which is determined by ground conditons irrespective of being a single storey building. Dont cut corners. It may bite you when or if you sell up later
You should not need planning permission as long as the area of the proposed building does not cover more than 50% of the original house area (which includes the garden). Furthermore if it is less than 3m high (4m for a pitched roof) you do not require planning permission.
With regards to building on the concrete base, please do not take short cuts !!. You do need proper foundations with trenches filled with concrete upto to at least 18 inches depth. I was advised by my bricklayer to dig upto 24 icnhes and to pin the concrete with steel rods at intervals. The soil can be very pliable and whilst you think there may not be much weight, you have to remember that the bricks/blocks also put a load on the foundation. A liitle bit of extra work now will hopefully prevent problems of subsidence or walls collapsing in the future.
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