Semi-detached house. 2nd floor on rear extension possible?

Suppose you have a 2 storey semi-detached house with a single storey semi attached extension at the rear (typically kitchen / bathroom), how easy is it to build a 2nd storey above the extension on one half of the semi?
What problems would you have to overcome?

On 10/12/2013 19:25, Michael Chare wrote:
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Do you mean that both houses currently have a single storey rear extension, separated by a party wall, and that you want to build a second storey on just one of them?
Are you asking from a physical construction point of view, or about planning issues, or both?

On 10/12/2013 19:33, Roger Mills wrote:
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Yes
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Both

On 10/12/2013 20:18, Michael Chare wrote:
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From a planning point of view, your second storey is going to be very visible from the upstairs of the other semi, and may block a lot of light - depending on orientation. So you need to discuss this with your neighbours, and take advice from the planners before going very far.
As far as physical construction is concerned, the wall above the existing party wall will be an outside wall, and will need to meet thermal efficiency standards - and will probably need to be quite a lot thicker than the party wall (cavity vs solid, etc.). So you'll probably need some steelwork on your side of the party wall to support the extra thickness. This pre-supposes, of course, that the foundations can support another storey - as mentioned by others.
Life would probably be easier if you could persuade your neighbour to mirror whatever you want to build. Some - but not all - of these issues would then disappear.

On 11/12/2013 20:06, Roger Mills wrote:
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+1