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Garden Room Extensions: Our Comprehensive Guide


I’m not sure if your household is like mine… a little chaotic from time to time?

When I took the big step to start working from home it made concentrating on work quite challenging.

To solve the problem, I first considered those giant earmuffs you see people wear through plane windows at airports.

After doing some research however, I realised how few earmuff colours were available and moved on to a better idea… a garden room extension!

I thought some of you may be in a similar position, so I have prepared a useful guide to the entire garden room extension process.

1. What is a garden room extension?

Garden Room Extension
Modern Home Gym Garden Extension

A garden room extension is usually a standalone building, detached from your house. Unlike a typical extension, garden room extensions are a little more sophisticated.

In terms of comfort, they offer the same levels as you would find in your home or extension. They will be heated, insulated, well lit, and can provide a versatile space depending on what you would like to use them for.

2. How much does a garden room extension cost?

Build QualityMaterialsMinMaxAverage Overall Cost
BasicPre fabricated timber£300/m2£400/m2£7,000
AverageBreeze block£1,500/m2£1,700/m2£32,000

There are a number of different factors affecting the cost of a garden room extension. The principal determining factor is the overall size or footprint.

In our experience, an average sized outbuilding measures approximately 4m x 5m (20m2).

To calculate the approximate construction cost, you would then need to work out the construction method and multiply that by the overall size.

So, for an average breeze block outbuilding measuring approx. 20m2, the cost would be:

20m2 x (£1,500 – £1,700) = £30,000 – £34,000

If your budget doesn’t stretch that far the same size outbuilding made of pre-fabricated timber would cost:

20m2 x (£300 – £400) = £6,000 – £8,000

3. What factors affect the cost of a garden room extension?

3.1. Overall Size

This is one of the main determining factors when trying to calculate how much your outbuilding or garden room extension might cost. Larger buildings will require more labour and more materials which adds to the cost.

3.2. Materials

There are a number of different construction methods and materials to choose from. The cheapest is an off the shelf design made of prefabricated timber. An intermediate design would be an outbuilding constructed of painted breeze blocks. The most expensive would be a bespoke brick-built outbuilding.

3.3. Foundations

This depends on your construction method. If you’re opting for a basic timber structure the foundations might not need to be so substantial. If you decide to go with a bespoke brick-built structure however, the foundations will be more complex and will require more materials and labour, adding to the overall cost.

3.4. Services

If your outbuilding is close to your house, there is less distance to travel in order to connect your new services such as heating or water. If you have quite a long garden and you would like your outbuilding to be out of the way, there will be more work for your builder which would increase the construction cost.

3.5. Fit out

As with all extensions, the quality of fixtures & fittings and materials can play a huge part in the overall cost. The use of the outbuilding is also relevant. For example, if you would like a home gym complete with bathroom and shower, then the cost of those materials and labour involved in fitting them can push up the price.

4. What can a garden room be used for?

The best part of outbuildings and garden extensions is that the space can be used for almost anything. Here are some of the most common uses:

Home Gym: Having a gym in your garden has so many benefits! You cut down on the commute, it’s open 24/7, and also has far fewer distractions.

Home Gym Outbuilding
Home Gym Outbuilding

Home Office:  Working from home is now part of everyday life. I have found that my productivity has increased significantly as there are fewer distractions. And, best of all, my 2 hour a day commute has been cut down to 30 seconds.

Home Office Garden Extension
Home Office Garden Extension

Kids’ Playroom: If your kids are like mine, they can make quite a bit of noise from time to time. Having their own play area gives them more freedom and means less tidying up in our house which is always appreciated.

Kids Garden Playroom
Kids Garden Playroom

Creative Space: If you like to draw, paint, dance or play a musical instrument, a garden extension is the perfect space to do what you love with few distractions.

Garden Artist Studio extension
Garden Artist Studio

5. Does a garden room extension need planning permission?

In most cases, if you own a house you would benefit from Permitted Development Rights. This means you can build certain types of extensions or outbuildings without the need of applying for planning permission.

With regard to outbuildings and garden extensions specifically, in order for your design to be considered as Permitted Development it would need to meet the following criteria:

Location: No outbuilding can be located on land forward of a wall forming the principal elevation (which is usually the front of your house).

Proximity to your fence: If your outbuilding will be located within 2m of your fence the maximum overall height is 2.5m (measured externally). If your outbuilding would be located further than 2m from your fence, there would be a maximum eaves height of 2.5 metres and maximum overall height of four metres with a dual pitched roof or three metres for any other roof.

Features: Outbuildings cannot have verandas, balconies or raised platforms.

Footprint Size: No more than half the area of land around the “original house” should be covered by extensions or other outbuildings.

Conservation Areas: If you live in a Conservation Area your outbuilding would require planning permission.

Restricted Sites: In National Parks, the Broads, Areas of Outstanding Natural Beauty and World Heritage Sites, the maximum area to be covered by buildings, enclosures, containers and pools more than 20 metres from the house should be limited to 10 square metres.

Planning Permission: If you do not meet the above criteria, your garden extension would require planning permission. Your Local Planning Authority will probably have a planning document called ‘Residential Design Guidance’ (this is where Google comes in) which will provide some guidance about the specific policies relating to outbuilding design and sizes in your borough.

6. Benefits of a garden room over the extension alternatives

Cost: Less expensive when compared to a traditional extension.

Construction Time: Quicker construction times for prefabricated structures (some can be put up in in 2-5 days).

Permitted Development vs Planning Permission: Most outbuildings or garden room extensions do not require planning permission.

Building Regulations: Depending on the type, in many cases you do not need building regulations approval.

Fewer Tradespeople: Many suppliers will construct their own outbuildings, so you don’t need a builder.

7. Useful garden room extension checklist

Use: The first step is to decide what you would like to use your outbuilding for. For example, if you would like a home gym, you might also want a shower which needs to be considered before you start designing. If you’re going for a home office, perhaps you need more plug sockets for laptops and printers?

Budget: We have set out some garden extension costs above. At this point, the most important step when planning your budget is to work out how big you would like your outbuilding to be, as this has the biggest impact on the overall cost. Once you have a rough size in mind, think about other factors such as quality of materials and fixtures and fittings.

Planning Permission: If you own a house, the chances are you would not require planning permission subject to meeting the relevant Permitted Development criteria. Another option is to book a free 30 min consultation with one of our architects who are always available to give advice and discuss your ideas.

Quote from a builder: Once you have a good idea of the type of outbuilding you would like, you can then approach different builders to obtain an estimate.

Ask for references: Always hire tradespeople with relevant experience and qualifications or accreditations. Ask to see examples of their work before agreeing to hire them.

8. Can a garden room extension add value to my home?

The short answer is, yes!

‘Nationwide’ recently carried out a survey and found that a high-quality garden room can boost the value of your property by 5% to 15%.

We also contacted several local estate agents who estimated that the increased in value would be approximately 1.5 times the cost of building the outbuilding.


Outbuildings are a fantastic and cost-effective way to get that little bit of extra living space. If you would like to discuss your ideas with one of our architects or planning consultants, please feel free to book a free 30 min consultation.

Our friendly team are always on hand to give advice and answer any questions you might have.

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