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A customer said to me yesterday that she’d heard that the sheds are on their way out. I reassured her that we remain committed to ShedLife around here. While our summerhouse specification has been replaced by brand-new garden rooms, the sheds are here to stay!

When we introduce a new product, there’s bound to be some confusion at first. 

That’s exactly why this blog exists. Informed Sheddies choose garden buildings that are perfect for them. But if you don’t know your cladding from your air cavity, how can you be expected to make the best choice?

So let’s explore the differences between our new U12m2 Garden Rooms and the Blackstone Garden Rooms we’ve been building for years. 

Once you’ve read this article, you’ll understand exactly how the two products differ, and if you’re considering an all-season garden building, you’ll know which one suits you best.


The most obvious difference between these two building types is right there in the name of the new product. If you’re looking for a Garden Room under 12m2, you’re looking at our brand new three-layer specification.

If you’re after a building larger than 12m2, you want a Blackstone Garden Room.

Why the difference? It’s all a matter of engineering. 

We can use the simplified specification for smaller buildings and ensure that they are still solid and stable enough to last. But to ensure the same structural stability in a building above 12m2, the five-layer specification is necessary. 

This is why we won’t use the three-layer spec for anything above 12m2. A larger building wouldn’t meet our high standards for stability and longevity. 

But for the smaller builds, it’s absolutely perfect.


When we realised that it was time to retire our summerhouse specification, we knew that we didn’t want to get rid of these beautiful buildings entirely.

To pay homage to our summerhouse heritage, we’ve preserved four of our designs to upgrade to our new U12m2 Garden Rooms. Each of these is available in three standard sizes.

The Rannoch, The Vorlich, The Kindrogan and The Deuchny live on, although they’re now so fauncy you might not recognise them!

While it’s still possible to customise these at an additional cost, our new Garden Rooms will be based on these four designs.

When it comes to a Blackstone Garden Room, however, you’re starting with a blank canvas. Each of these buildings is completely bespoke and customised to your specific requirements. 

We project-manage every aspect of a Blackstone Garden Room to ensure that your building fits your budget, your needs, and your unique timber building dreams.


Now for the technical bit!

We’ve learned over the years from our Garages that the key to keeping water out of a timber building is LAYERS.

But how many layers does a timber Garden Room need?

As I mentioned before, the answer to this question very much depends on the size of the building.

U12m2 Three-Layer Wall Specification

A diagram of the three-layer wall construction of a Gillies & Mackay U12m2 Garden Room

Our new Garden Rooms have three layers, plus a vital air cavity to prevent moisture ingress. 

Unlike a single-skinned building, any moisture that penetrates the outer layer of cladding is stopped in its tracks by that air cavity. 

The air cavity, the insulation board and the timber lining ensure that the building stays cosy when it’s cold outside, and cool when it’s clement. 

In these smaller buildings, this specification creates a solid, durable Garden Room that can be used all year round (with the addition of a heating source).

U12m2 Cladding

And have I told you about our super-cool new cladding yet?

Radiata Pine ThermoWood is a stable, moisture-resistant timber, with even fewer knots and more regular grain than Scandinavian Redwood. It’s also seriously good-looking. 

Thermal treatment doesn’t just protect the wood against moisture, insect damage and rot. It also prevents movement in the timber, meaning that your building stays as square and solid as the day it was installed. 

This protection means the wood can be left unpainted for a natural finish. 

Or, if colours are your thing, it takes paint beautifully. 

Isn’t it great to have options?!

U12m2 Doors and Windows

Our new Garden Rooms sport gorgeous anthracite grey aluminium double-glazed windows and doors. 

Slim and refined, these units complement the designs of the buildings perfectly. Each design comes with a set of double doors and two windows as standard. Additional windows can be added as an upgrade. 

As well as looking the part, these doors and windows confer all the usual benefits of double glazing. Your building will stay dry and warm, and whether you want to block out the noise of the outside world or create some noise of your own in there, everyone gets peace and quiet.

U12m2 Roofing Materials

Two of our new Garden Room designs come with steel box profile roofing.

We’ve written before about how much we love steel box profile roofing – a 40-year materials guarantee is not to be sniffed at! 

Properly installed, a steel roof has decades of life in it, and it’s virtually maintenance-free. That’s why we’ve chosen steel for our Rannoch and Vorlich designs.

But what do you use when you have a shaped roof?

Our Deuchny and Kindrogan designs needed something to rival steel’s durability, but also provide the sophisticated finish needed for such refined buildings.

Enter EcoSlate.

With the aesthetics of a traditional slate roof and the dependability that comes from cutting-edge product development, EcoSlate is the perfect roofing solution for our new shaped Garden Rooms. 

Made from recycled plastic, EcoSlate has a materials guarantee of 50 years (50! Count them!) and can withstand winds of up to 110mph. 

U12m2 Base Requirements 

We all know how important the base of a garden building is. A great building starts with a great base.

For our new Garden Rooms, the simplified specification means that level concrete slabs or ground screws form the ideal base. 

With this sort of specification, you can have your perfect home office, reading room, garden party pad – whatever you like, really!

Blackstone Garden Room Five-Layer Wall Specification.

A diagram of the 5-layer Blackstone Garden Room wall construction.

But what if you’re looking for a larger Garden Room that’s essentially a detached extension of your home?

This is where our Blackstone Garden Rooms come in. The five-layer wall specification used for these larger buildings is fully compliant with Building Standards legislation. This means that a BGR is as safe, fire-resistant, waterproof and structurally sound as your home is – by law!

If your Garden Room is going to be an extra bedroom, an Airbnb, or a holiday home, this five-layer specification is essential. 

As well as the outer cladding, air cavity, insulation and lining, a Blackstone Garden Room has a breather membrane for even greater moisture protection, and a layer of OSB for added structural stability and insulation.

They’re perfectly designed living spaces, studios, business premises… The list goes on!

Blackstone Garden Room Cladding

A closeup of painted weatherboard, against a background of a painted weatherboard panel on a garden room.

Our BGRs come with pressure-treated Scandinavian Redwood weatherboard cladding as standard.

This cladding must be painted with a water-based exterior paint – we recommend Sadolin.

Radiata Pine ThermoWood is also available as an upgrade which replaces the previously offered Siberian Larch cladding upgrade. 

Like Siberian Larch, Radiata ThermoWood can be UV-protected to retain its original finish. Or it can be left untreated to weather to a silvery-grey colour. 

Blackstone Garden Room Doors and Windows

These larger buildings are more suited to the proportions of uPVC doors and windows, and that’s exactly what we use. Energy-rated, long-lasting, reliable – what’s not to love?

We’re back to that custom life again – the number, placement, shape and size of your windows and doors will entirely depend on your budget, your needs and your wants. 

If you’re building a photography studio you may want to make the most of the natural light. A cinema room or beauty salon would be completely different projects. That’s why the first part of your consultation involves getting to know exactly what your plans are. 

Blackstone Garden Room Roofing Materials

A bespoke building gives you additional choices. 

The roofing materials on our new Garden Rooms can’t be switched, but with a BGR you have more room to customise your building. 

Our Blackstone Garden Rooms come with steel box profile roofing as standard. But if you’re looking for added sophistication, EcoSlate is available as an upgrade on apex BGRs. 

EcoSlate’s design is unsuitable for pent roofs, so a pent Blackstone Garden Room will have a steel box profile roof.

Blackstone Garden Room Base Requirements

A poured concrete plinth for a garage or garden room base

A Blackstone Garden Room is a serious building that needs a serious base. 

A good base for your building will keep it standing firm for decades to come. For a BGR, this means either a poured concrete plinth or ground screws.

It’s over 12m2, it’s fully compliant with Building Standards, and it’s designed to last a lifetime. Oh, and it’s created according to your requirements. Each Blackstone Garden Room is a totally unique project, brought to life just for you, the way you want it. 


So, how much is all this going to cost?

There’s definitely a difference in price between our U12m2 Garden Rooms and our Blackstone Garden Rooms, and various factors that affect this.

Let’s start by looking at the prices of our new Garden Rooms.

How Much Is An U12m2 Garden Room?

From £8,997 to £12,697 for a Rannoch or Vorlich, these smaller Garden Rooms are much more affordable than the custom Blackstone Garden Rooms.

The Kindrogan and Deuchny are between £9,897 and £13,397, depending on the size you choose. 

Additional extras listed on our product page include laminate flooring and guttering. 

Beyond this, customising our new Garden Rooms is absolutely possible, but these customisations will be charged separately. 

What Other Costs Are Involved?

The two main additional costs involved are the base and an electricity supply. 

We have a list of recommended contractors for groundwork, depending on where you’re located. There are also estimates for this work available here.

RC Electrical covers Tayside and Fife and was good enough to give us estimated prices for installing electrics in your building.

Of course, these estimates will depend on your individual project and circumstances. Lighting and heating are necessary for year-round use, but you may also need WiFi, and AV equipment or have other special requests. 

How Much Is A Blackstone Garden Room?

A large apex roof Gillies & Mackay Blackstone Garden Room

Because each project is unique, there’s no single figure I can give you that neatly answers this question.

However, roughly speaking, BGRs start at around £18,000. Depending on the size and complexity of the project, they can cost as much as £60,000 and beyond.

You can find guide prices here.

What Other Costs Are Involved?

Groundworks and electrics are additional costs completed by external contractors. Again, these costs will vary depending on your individual project.

Contractors will carry out site visits and consult with you to make sure that you’re fully aware of your options and the costs involved. 

We also project-manage each BGR to ensure that everything runs smoothly and coordinate with your chosen contractors to schedule everything appropriately. 

Where Can I Find Out More About Gillies & Mackay Garden Rooms?

We’ve been building Blackstone Garden Rooms for years! There’s tons of information in our Learning Centre about these large, custom buildings.

Our Show Area also includes a beautiful BGR that you can come along and see.

As for our U12m2 Garden Rooms, you’ll be able to visit and inspect these brand-new buildings from mid-November. Big changes are afoot!
Don’t miss out on our introductory offer – save £500 on any U12m2 Garden Room if you place your order before the end of 2023. Get in touch with our Sales Team to find out more.

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