It is however never a simple answer with several factors influencing the final construction project costs.
A rough example based on £ per m²
If we look at this simply to start with you can work it out with a rough budget cost on £ per m². A few years ago this was pretty simple maths at £1000 per m². General build costs have risen and it would probably be more advisable to allow £1200 per m². So, for example, if you are building a 3m by 4m single storey extension this would come out at £14,400 (3 x 4 x 1200).
If you are planning a 2 storey extension the upper floor would be an additional half of the £ per m². This is because much of the building work required for 1 storey is only needed once, regardless of storey number, such as the foundations and roof. So if the same extension was 2 storey it would come out at £21,600 (3 x 4 x 1200 x 1.5).
This figure covers the general build including basic plumbing and electrics to a reasonable standard but not expensive finishes such as tiling. If you are looking for a high end finish this will increase the costs per m², and if your extension includes a kitchen or bathroom these will be extra too.
Other influences on the cost of an architectural project
Other factors to take into account which are hard to put a figure on are those that are site and/or project specific. For example;
- 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 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.
- Unexpected costs. For example, moving a gas meter might seem like a minor thing but can easily cost £500 – £1000. Connecting into your existing drainage may not be straight-forward and require more work than expected.
- Off-site project costs. Fees for architects, engineers, planning and building regulations etc. These can add on a possible 10% – 20% on top of the build costs.
What contingency is recommended for my construction project?
A good sized contingency is recommended on any project; 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 with, the more accurate their cost estimates will be.