House extension cost calculator or estimator
Many home owners would like some sort of house extension cost calculator or estimator for their
preliminary budgeting but in reality this sort of thing does not exist that would provide any real degree of
accuracy. Each house extension project is pretty much bespoke to suit the site conditions, size and scale of
project, extent of overlapping works, external materials used and quality of internal finish.
Therefore it is not possible to have a ‘one size fits all’ house extension cost calculator
or estimator. However, the RICS do have a basic cost per square metre for the main build of most house extensions
that could be utilised with some tweaking. Your best way to having a house extension cost calculator is to use an
experienced house extension building designer who should be able to give you a framework or some figures of what
your intended house extension costs may be.
It is not an exact science estimating the cost of any building works and even when you do go out for tender
prices based upon even the most details and well scheduled plans, there could still be many thousands of pounds
difference between the 3 builders tender prices.
Therefore another factor is the builders margin and each builder will have their own assessment of the gross
profit they need to make to enable the business to succeed. When house extension work is tight many builders may
lower their margins to be competitive or to give themselves the best chance of securing the work if they really
need a new scheme to work on. Other very busy builders may be happy maintaining a gross 24% margin purely because
they are pricing more work than they can handle and are confident of obtaining something when required.
So, use any house extension cost calculator or estimator you may find with extreme caution.
Simply use it as one of many tools and ways to obtain estimated house extension building costs. Do not fully rely
on any one method.