Likely Extension / Building Cost?

I'd be grateful if anyone could give me an idea of the cost involved with building an extension on my house.
I own a victorian terraced house in the North East. The kitchen and bathroom are located at the back of the house on the ground floor. While the house itself was built in 1899, this part of the house was probably built in the late sixties / early seventies (guess). It's a flat roof structure covered with asphalt, probably about 3m wide by about 10m long (estimates).
Stairs to the first floor are split with a small landing before the stairs turn back on themselves to the first floor. On this landing there's a large window which directly overlooks the roof of the kitchen / bathroom. It's probably about 6 foot high and about 4 foot from the top of the kitchen roof.
My Idea was to build ontop of the kitchen and bathroom, creating an additional bathroom upstairs, accessible from this small landing on the stairs. Ideally I would like the new work to extend the full length of the kitchen and bathroom on the ground floor. Other houses in the street have a mixture of full length and half length extensions. Building this way should mean I don't have any problem getting to the waste pipes currently used in the Bathroom.
The first thing I'd need to find out is how good the foundations are but before I start any ball's rolling I'd be grateful for an idea of likely costs - this could be completely unaffordable.
Cheers,
Rob.
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Work around nine hundred to a thousand pounds per "cubic meter" should give you a very close amount.
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