The cost of a single-story expansion will vary depending on the size of your project and where you reside in the nation. A simple expansion will normally cost approximately PS1,000 per square metre, so that a 3x5 metre single-story extension will cost roughly PS15,000. Multiply this figure by the number of square metres you need extended and you'll be able to work out how much the project will cost. The national average price for a single-story extension is £10/m², with costs ranging from £5/m² to £20/m². The price will depend on such factors as the type of building material you choose, the size of the room you want added and the location of the house. Extension prices are generally higher if you build into a hillside or require structural changes to an existing part of the house. Single-story extensions can be done in one phase or over several months. If you plan to do the work over several months it's important to allocate enough money for maintenance during that time. You should also consider how you will deal with any changes of mind or necessity during the construction process. For example, you might want to add a bedroom later when your family grows and cannot find space elsewhere in the house. However, because you cannot increase the height of a single story addition, this would mean giving up some of the attic storage area which was very useful when you first moved in.