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@Bobbo
There is a hell of a lot more building involved.
Added to that costs only become predictable once you're out of the ground. A couple of years ago I was working on an extension and refurb project in West London, the groundworkers were sinking the foundations for the extension when they found a water main running through the back garden. This is not uncommon as after the war the priority for the water board was repairing and covering up services quickly and they often weren't mapped. The project had to be put on hold while Thames water got their act together to inspect, survey and map the water main which took weeks the cost of which had to be met by the client. It consumed their entire £30k contingency. It is still better than the other problem you occasionally get left over from the war; finding unexploded munitions.
A lot of less scrupulous contractors will add a fairly large sum to the cost of groundworks to cover the fact they are unpredictable but then pocket the extra if all goes well.
It may be possible to fill in a side return for less than stated if you are not after open plan areas but given that you are already thinking of skylights I'd assume this isn't for you.