How Much Does a House Extension Cost to Build? [& Value Added]

This Extension Guide is so you know exactly what you want, how to get it – and pay the right price for it!

Websites have popped up in the last few years like rated people or checkatrade, taking some of the pain out of finding reputable people and companies to carry out building work. 

But there still is that grey area with larger jobs like extensions and loft or garage conversions. Here you have to place an extremely large amount of trust in people. It’s your money after all, but it’s more importantly also your home. 

The guide is designed specifically, so you are fully armed with all the right information to take on the builders! It’s so you can speak the same language and find out answers to questions you may think are silly or be embarrassed to ask. 

We are going to run through all the basics from the types of extensions which are suitable for your property type, the costings of those extensions and how they are calculated. Whether quality of materials is something to quibble and whether you could be subject to planning laws?

Short on time – here’s the Bottom Line!

  1. Single storey extensions typically cost about £1,700 per square metre whilst two storey extensions cost around £2,700 per square metre. 
  2. Keep an extension within 50% of the original floorspace of the house and the new roof, no higher than the original and you could avoid the dreaded planning permission! 
  3. With a 2 storey extension of around 20m2 in size you could double the size of an average kitchen and create another master bedroom upstairs for circa £54,000.
  4. Architects’ fees are usually around 10% of the extension cost. Factor that in and agree with the builder [beforehand!] who is responsible for those costs.
  5. Adding additional living space and a new bedroom can add 20-25+% on to the value of your home! Agree everything with a builder before accepting any written quotation, these points include [but aren’t limited too] the total cost, start date, specs and finish date! 

Try and hold 5-10% of the extension cost in reserve in case you come across any unforeseen nasty surprises.  

Costs for Different Types of Extensions

Single Storey Extension

A single storey extension is simply an extension of the home on a single level, usually the ground floor but this term also can include loft or attic extensions. Basement extensions, below ground level are examples of single storey extensions too.

Ground floor extensions are typically the ‘easiest’ to build and they are most commonly on the rear and or side of a property.

Costings for single storey extensions are estimated at £1400-£2000 per square metre. In London and areas of the south east of England costings for single storey extensions are estimated at anywhere between £1800-£2500 per sqm. 

Lets run through those figures with an example:

The average living room is about 17.5m2 so increasing your living space by 50%, equallying 8.75m2, would likely cost £12,250- £17,500. 

Let’s say you want to double the size of your kitchen? Well the average kitchen is 13.5m2 so adding a 13.5m2 extension onto your home would cost you approximately £18,900-£27,000. 

Bungalow Extension

A bungalow is typically defined as a single storey dwelling, so any bungalow extension would be classed as a single storey extension so are included within single storey extension costings, as above.

Two-Storey Extension

Two storey extensions as the name suggests are 2 floor extensions, usually, but not limited too, a ground floor and a first floor. 

Costings for two-storey extensions are estimated at £2,000-£3,400 per square metre, however in London and areas of the south east of England costings are generally higher at between £2,400-£4,000.

The average kitchen size and master bedroom size are each about 13.5m2. So an extension to increase the size of these by 50% would equate to an extension of around 13.5m2 and so would cost £27,000-£45,900. 

If adding only 50% to the size of your living space is not enough, or you have a larger budget to create a bigger ‘snazzier’ space then read on…

Doubling the size of an average kitchen (i.e adding a 13.5m2 extension) & creating a new bedroom at 13.5m2, [27m2 total extension size]  would cost you somewhere in the region of £54,000-£91,800.

Wow, think of all that extra space…

Extensions That Include Bathrooms & Kitchens

To be clear builders use the terminology ‘finished’ to mean extensions that include everything within the price. So they are referring to an extension that has been built and then everything is installed and fitted inside as well as out. i.e the job totally finished. 

Usually ‘finished’ refers to the installation of  kitchens and or bathrooms but it can, if specified, include things even like log burners.

All items [and the spec of those items] within a finished price must be agreed upon prior to the acceptance of the quotation by the customer. This is so the builder can obtain the prices of the desired items, purchase them if necessary and then quote this within the finished price.

What Affects The Cost of an Extension?


Well how big do you want it? The larger the extension the more expensive it will cost. The higher the specification of things like the kitchen or the appliances again the greater the cost. 


Your location has a large impact on the extension cost. On average London has the most expensive property prices and labour costs in the UK. As such builders command the highest extension prices.

Quality of Materials

The materials used can have the largest impact on costs, for instance oak beams or bespoke windows and doors. Handmade fitted kitchens or handmade imported bath tubs will all rack up the price. 

But prices for things less thought about like brick type or style will also have a financial impact and affect the aesthetics of the build too. 

For instance reclaimed bricks are often 2-5 times more expensive than new house bricks. Reclaimed bricks or ‘second hand’ bricks are often used when trying to match the extension to the existing brick style of the property. All things to bear in mind.    

Roof type

It’s the old ‘flat roof vs pitched roof’ argument. Flat roofs are generally cheaper but don’t last as long. A pitched roof could cost more but has the potential to be converted into living space at a later date and they tend to last longer than a flat roof. It’s all dependent on your budget…


Opening up the side or rear of your home to allow an extension to be built is typically done between March and October to allow for the best weather. Opening up the roof during the middle of winter, whilst possible, can host many issues.

Therefore builders are in greater demand at certain times of the year, especially specialist builders, ones who just specialise in one aspect of building work – like loft extensions. Start all your research early to maximise your chances of success! 


New baby on the way, mum and/or dad coming to live with you, or just a pokey kitchen? Whatever the reason for the extension, do you require it done ASAP? Well this could be a reason that a builder tries to hike up the price. Keep your cards close to your chest or agree a deadline with penalty clauses!

Building Up vs Building Out

Loft extensions are so popular because instead of ‘building out’ and losing say patio or garden space people can just build up! Converting attic or loft spaces is often a fraction of the cost of even moving house and if it adds another bedroom you’ll probably increase the value of your home by 25%. Win Win!

Extension Cost FAQ

How much value does an extension add?

It can be thousands if done well! Although it isn’t always quantifiable, as the resale value of any home is subjective. 

Ask yourself what are people who might buy your home looking for? 

Is there a space trade off? Does an extension mean more living space but less outdoor space for instance…? 

Assess your property with fresh eyes. Generally anything that is smaller by comparison to the rest of the house, is the first thing people consider extending. For instance a 3/4 bedroom house with a smaller family bathroom highlights an ideal extension senario. 

The sweet spot with extensions is typically a two storey extension. So downstairs you create a larger living space or kitchen and upstairs you create a new bedroom. 

Creating another bedroom can add serious value. Just look at any new build development and look at the prices. Often the difference between just 1 bedroom is huge. Adding more living space in any property can create a massive uplift in the value.  

Typical extensions usually add around 25% to the value of your home.

Can I extend my property without planning permission?

Usually you will only need planning permission if the extension is: 

  • Larger than half the original footprint of your home
  • The extended roof or eaves are higher than the original roof or original eaves
  • Work includes changes to the existing roof
  • Includes a balcony or veranda
  • Includes a new chimney, flue, soil or vent pipe

Source: Federation of Master Builders

A great place to visit, for more planning permission information, is the planning portal website:

How long does it take to build an extension?

For small simple, single storey extensions it can be just a matter of weeks. Up to 4 months for larger, more complex builds.

A 2 storey extension will usually take no longer than 3-7 months. However it could be longer depending on the weather.  

How much does an architect cost for extension plans?

Don’t forget that architects fee – that’s usually around 10% of the total build cost.They usually range though from 6-12% of the build cost. When you instruct a builder to carry out an extension be sure to know who is paying for this and what the fee is. This will save any awkwardness later down the line. Remember all fees could be negotiated! 

Will a single storey extension add value?

A professionally carried out single story extension could add significant value to your home. It’s important to do your market research to find out more. Research could include going to Rightmove or Zoopla and finding out what the sold prices of homes are, like yours, after an extension. This will give you an instant indication of whether or not it’s ‘worth it’ and how much value you could stand to add.

In Summary

Don’t want to faff around, waiting in all day, for a tradesman or company to come and give you a quote, to then have to repeat the process twice over just to obtain 3 quotes… 

To then not really know anything about the companies or people you’re handing money over to? To then get multiple quotes online instantly.