Is There....

[CailinDeas]

a guide price per square foot for building a very ordinary home extension?

[bookbinder]

You might find something here:
http://forums.moneysavingexpert.com/showthread.php?t=317959
http://www.mybuilder.com/questions/v/4472/how-much-would-it-cost-to-build-a-3m-x-9m-single-storey-extension-to-a-semi-detached-house-approximatley
http://www.jewson.co.uk/working-with-you/for-self-builders/preliminary-planning/calculators/build-cost-calculator/
http://www.whatprice.co.uk/building/home-extension.html

[The Builder]

Bookers' link is very good for a rough idea, Cailin.

The short answer though is that there are far too many variables. Prices per square foot/metre are used mainly for insurance purposes. They tend to be on the high side to cover all eventualities.

They can be quite misleading I'm afraid.

[CailinDeas]

Thank you both.