Extensions Versus Annexes?

We often get asked the question, “Can you build one of your annexes attached to a house?” and the simple answer is “Yes.”

There are several key differences in building an annexe and building an extension that you may want to consider before deciding whether or not an extension or an annexe is the right solution for you.

There is no doubt that thanks to the relaxed planning laws with regards to permitted development the whole process of adding a single storey extension in particular to the rear of your house (depending on the size of your plot and any existing extensions or outbuildings you already have) is much easier than ever before.

Being able to build an extension that would in most cases completely transform any home into a modern living space like you see on TV shows like DIY SOS for example is very appealing.

So here are some of the main differences between building an extension and building an annexe in the garden.

Feature Article: Extensions Versus Annexes?

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The Most Popular Extension Is Adding A Kitchen

Whether your house is a  town house with a long narrow garden or yo are lucky enough to have a detached house where you can extend across the full width of your house, the most popular way to transform your home and create a modern open plan living space is to add an extension like the one shown above.

With an extension you will require Building Regulations even if you do not require planning permission and you can expect to pay VAT @20% on the contracted price. Most building companies will charge a similar rate of anywhere between £1000 & £1500 per square metre depending on the style and specification of the building.

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Another Popular Upgrade Is To Transform Your Existing Conservatory

For a decade or two conservatories enjoyed all the benefits now associated with the new permitted development laws where if there was a door between the house and a conservatory and it was not deemed to be a habitable room, planning would not be required.

What that allowed many of us to do was to build conservatories as extensions at a very reasonable cost. As the popularity grew (and we ran a bespoke conservatory company at the time) we found people started to frequently ask us about conservatories that could be less glass / more brick and if possible could they have a tiled roof?

At that point they too had to be covered by Building Regulations and just as you will find today if you are planning an extension, you will find the baseworks required to comply to Building Regulations will more than likely be far deeper than those of your house.

You will also need to factor in waste disposal costs as skips to remove the soil that comes out of the foundations can easily rack up a few thousand pounds at an average cost of £250 per skip.

The main benefit of creating an extension to your home or transforming your existing conservatory is that it becomes an integral part of your home.

You may find you need to upgrade your boiler to cope with the additional radiators and you may need to upgrade your electric metre to cope with the extra demand.

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Skylights And Lantern Tops Add Extra Light

Whether you are building an annexe or an extension, adding sky lights or a lantern top (a small conservatory style roof) to the extension roof is a great way to add more light to your room that will look attractive and add value to your home.

With one of our custom designed annexes, this sort of feature can be an integral part of the design although the width of the skylights will be determined by the structural calculations of the trusses used in the roof. 

Whether you are building an annexe or an extension, features such as the popular bi-fold doors can also be easily incorporated.

You will find when you use Google searches for home extensions that the cost will vary dramatically depending on the design. The simpler the design (for example a sloping roof, patio door to the front and one small window to the side will help keep the cost to a minimum)

The unknown factor with an extension will be the footings and foundations although in some circumstances the swift foundations system that we use on our annexes can be used on extensions too.


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Annexes Are Considered Dwellings And Will Always Require Planning Permission

We know there are companies out there suggesting that you can build an annexe in your garden and provided you eat all your meals in the main house you will not require planning permission.

We disagree and so will your Local Planning Authority.

The fact is that is someone is going to be living in an annexe it will be considered a dwelling and ALL dwellings regardless of size, type of style require planning permission.

Trying to bypass this part of the process is storing up problems for the future if you ever decide to sell the main house just as you will find building an extension that does not come under the permitted development laws would. It will become a major stumbling block to selling your home.

Adapting an extension for a dependent relative however will come under Building Regulations for the extension NOT planning permission.

Built as a mobile home, currently residential annexes in the garden are considered zero rated for VAT purposes. 

General Considerations

Building a self-contained annexe in the garden will in 99% of the cases add more value to your home than just adding an extension.

In terms of cost, an extension can cost as much below the ground as above and can easily cost up to 60% of the cost of an annexe.

An annexe is the only solution for a family member being able to live in the garden on a permanent basis and can only ever be sold as part of the main house, not independently.

Planning permission may not be required for a single storey extension, granny annexes will ALWAYS require planning permission which takes approximately 8 weeks (we offer this service as part of our contracts and currently have a 99% success rate) 

When thinking about dependent relative living an annexe would be great for special needs when you need to provide care for someone, for ageing parents looking to downsize and wanting to retain independent living, an annexe is the best option.


  1. Check with your Local Planning Authority for the latest permitted development laws in your area.
  2. Planning permission will be required for all annexes, they are classed as dwellings and are currently zero rated for VAT purposes only when built as a mobile home.
  3. We have a 99% success rate in obtaining planning permission for you.
  4. Extensions can cost anywhere up to 60% of the total cost of an annexe, and annexes will always add more value to your home and make it more attractive to future buyers.
  5. All internal and external specification is of the highest quality available and we offer the widest selection of kitchens and bathrooms in the industry whether you are building an extension or an annexe.

As always we look forward to your thoughts and comments below or join in the conversation on our Facebook Page !

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About The Author

Diane Cossie

Diane Cossie is a blogger and social media enthusiast who is the face behind our popular Facebook page, creator of our website and one of our annexe and specialists.

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