By far the most common type of extension work across the UK and Manchester, a rear extension normally provides the most useful practical space for a property which uses the empty space of a back garden.
Generally speaking, the cost of these rear extensions will ranges from £1,800 - £2,500 per square meter. the exact number will be lower or higher depending on structural changes and cost of glazing and fittings (for instance, an expensive bifold door instead of a regular external door).
You can use the calculator below this paragraph to make a useful estimation for your plans. Generally speaking - normal single storey rear extensions will cost between £30,000 - £45,000. Regular double storey rear extensions will cost £60,000 - £75,000.
These figures are for the construction cost, bare in mind also the cost of an Architect, Structural Engineer, Building Control Officer and application fees such as a planning application. A general rule of thumb for this is 10%.
Image credit: @A9architecture