It’s notoriously difficult to provide cost estimates for work to existing homes, including extensions. Usually, as previously stated, the comparative cost per square metre is higher. The measurement of cost per square metre is an industry standard that you should get used to seeing in your calculations — especially in the initial planning period.
There are two reasons for the difficulty in providing accurate cost estimates for work on existing properties. First, the work involved in making the existing house “better” is complicated and tough to predict. Until you begin the process and potentially start to uncover the need for remedial works to the existing structure, you never know the true cost of the project.
Second, over a whole house, the most expensive elements of a typical metres squared cost estimate — the foundations, roof, more expensive rooms like the kitchen
and bathroom — tend to even out over a larger project.
An extension has all of these things and often involves the most expensive rooms. This is why typical extension costs can be around £2,000 per metre squared and more. Remember, you will likely pay VAT on top of this so the cost is actually £2,400 per metre squared.
At this point, you will be asking yourself whether the renovation is worth it. Is a self-build worth it? In most cases where a property is in need of large-scale remedial and remodelling work, the answer is yes.
To compare figures, the average self-build project costs £1,303 per metre squared. To put this into perspective, you should be able to build a 140m² new house for the same price as a 75m² extension (£180,000).