The type of construction method and materials you use can have a big impact. Using non-traditional construction techniques could increase the costs, as could say using stone instead of bricks.
The site itself could be difficult to build on, if the extension requires additional ground works such as deep foundations this will add more to the construction costs.
There could also be small things that on the face of it don’t seem a big issue but can be costly; For example moving a gas meter can easily be £500 – £1000, or connecting into your existing drainage may not be straight forward and require more work than expected.
Away from the construction there are other costs you will incur as part of your project. Fees for architects, engineers, planning and building regulations etc can add on a possible 10% – 20% on top of the build costs.
Finally, it is always wise to have a good size contingency, 10% is normally advisable. Hopefully it won’t be needed but it is best to be prepared as working with an existing building in particular can throw up some unexpected issues that will need to be resolved.
If you are thinking of an extension, or any building work, it is best to speak with an Architect who can advise you on the specifics with your project. A good Architect will also provide you with alternative designs that can reduce your build costs while still getting the result you require. Most builders will give a free quote for the works, but the more information and detailed drawings you can provide them the more accurate their costs will be.