Extension Calculator - Your Guide to the Cost of Building an Extension

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Cost/Price of Having an Extension Built

pricing an extension
Assuming that your name isn’t Olly Garch, Croesus or even Gates, the thought process behind getting several quotations for your new extension is simple. You want to know that the price you will eventually end up paying for your construction project is a competitive and realistic one. Being a builder myself, I often meet up with a potential client to quote for their new extension and they will casually drop into the conversation that I am the third or fourth builder that has come to see the job. In effect, what they are saying to me is “You had better come up with a competitive price if you want to get the contract.” That’s fair enough and that’s what I would expect. But how do you know what is a competitive price? And how does a builder come up with that price?

The Benefit of Experience

Having completed numerous extensions both on my own and as part of a team, I will generally have an approximate figure in my head by the time I am walking back to the van, having seen the site and the plans. Most builders have a fixed amount per metre squared that they will charge for extension work. To this, the builder will have to take into account, and allow for, any additions and variables ranging from access to site, roof structure, fixtures and fittings, is there a knock-through? Is there any steelwork? Do I have to move any drains? Will the customer provide biscuits? Does this vest make me look a bit like Freddie Mercury? That sort of thing. But come here. There’s more.

First Impressions

This first meeting with your builder will give you a good opportunity to size him up. Do you like the cut of his jib? Or does he seem to be a frightful bounder? But rest assured he will not be the only one under scrutiny and he will also be having a good look at you too. Most people have seen the Builders From Hell T.V. series and such like, but from the builder’s perspective, it certainly cuts both ways, ranging from the superior customer who look at you as if you are something they have scraped off the sole of their shoe just because you drive a van and must therefore be educationally sub-normal, to the Victorian school mistress who lectures the cowering builder on the best way to do the job and the repercussions should he fail to implement her plans. “And stand up straight when I’m talking to you!” That’s without even mentioning the bloke that has been watching too much Tommy Walsh and thinks he is a fully qualified expert. His pristine B & Q toolset will tell a different story. He’s even got an orange hammer for Heaven's sake!

Spoilt for Choice

The good news from the customer’s point of view is that due to the current global financial predicament, there will be many builders out there who are desperate for your business. Polish and Eastern bloc builders have entered the market and there currently is a huge choice of builders and building companies to choose from. The builder you choose to employ may be a small contractor with two or three employees or they may be a larger contractor, but you will need to provide both with lots of information from the outset before he can provide you with a price. They will need to see full specifications and drawings before any figures can be arrived at with any accuracy.

The Secret Variables

You may notice that when the quotations start to drop onto your doormat, the prices contained within, vary considerably. This can be down to a variety of reasons. And I bet you haven’t even considered half of them. If a builder is really busy and doesn’t need or want to take on your job, he will bump up your price. If he thinks you will be a nightmare to work for, he will bump up your price. If he thinks the project is tricky or complex, he will bump up your price. There are builders out there who will look at the Mercedes in the driveway of your mock Tudor in Surbiton and guess what? He will bump up your price. But rest assured, it’s not all doom and gloom. There are many decent and hard-working builders who just want to earn a decent wage for a decent job and let’s hope you find one of those. They are certainly out there. The larger building firms will tend to be more expensive as they will have more overheads, they may use a quantity surveyors to price up the work and they may charge more for labour rates. Try and take all these things into account when making your decision.

Does He Charge Like a Wounded Bull?

So that’s cleared all that up then. Clear as mud. From my own perspective, I will charge on a cost per square metre basis for an extension. This will give me a base price onto which I will add the variables that we spoke about earlier. Hopefully, my price will then be competitive, but provide me with an acceptable wage for my efforts. It takes a lot of patience, phone calls and thought to competitively price up an extension and a builder cant just guess and hope he is in the right area. By following these principles, my price should be about right and, fingers crossed, I will get the job.


Hopefully your quotations will all be fairly similar price-wise and this will enable you to get a good feel for what you should be expecting to pay. A word of caution though: don’t automatically pick the cheapest quotation. Some builders will give you a price that seems too good to be true. There’s a reason for that. They hope you accept, get their foot in the door, and bump up the price in unexpected and unanticipated ways. A detailed and concise quotation encompassing stage payments should eliminate any misunderstandings. Good Luck!