Double storey extensions have the benefit of extending on the first floor as well as the ground floor. Ground floor new extended area is usually for a new utility room, reception area or even a down stairs bedroom with en-suite. Up stairs extended area is usually for bedrooms and maybe a new bathroom. Please note that designing and building a double storey side extension first will then affect the option of building a loft conversion. Permitted development volumes take into account the additional roof being added to your house, so for an example if you build a double storey extension and the roof volume calculates to say 20smq3 then you will only be allowed to build an extra 30sqm3 making the total 50sqm3. This will affect the size of your proposed loft conversion drastically and in some cases not allow you to have a loft conversion built to your specification. However if you build the loft conversion first then alloy for planning for the double storey extension you will get a positive result as the guidelines are set out differently and not by volumes. I have had clients call me saying they would like to have a loft conversion and i have to break the bad new in same cases saying that the loft is not possible because most of the volume needed to achieve a decent size loft has been used up on the roof being made up on a passed double storey extension design. This can actually reduce the value of your house and the potential to extend creating the extra bedroom has been lost.