This is an easier one to answer if you are adding an extension to a bungalow!
I use a picture of foundations because when you are building a single storey extension you absorb the groundworks in a single level, but on a two storey extension your costs are spread.
I could have used a picture of a roof too because the cost of building the roof is absorbed fully in a single storey extension and shared across a two story extension.
Generally what this means is that the difference between single to two story is 50% more. So If your single storey extension was £40,000 your two story extension is £60,000.
If you can use more space upstairs and downstairs then you adding more square feet at a cheaper price with the two storey extension.