That's because it isn't 8 x 8, it's a 4 x 4 extension (ie doubling the size of the existing kitchen) and being an accountant, I don't understand basic arithmetic and doubled both dimensions when I should have only doubled one. The actual floor space including the extension will end up 8 x4.
To...
I have just been quoted £85k minimum (ex VAT) for similar for an extension from a kitchen (knock through, steel etc) into an existing conservatory to be replaced by a roofed extension. That was just for the construction. It did not include the buying and fitting of a new kitchen, appliances etc...