Garden room project

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I'm gradually finding my feet with SketchUp. Very early days so far and I'm sure there's things I'm not doing as quickly and accurately as possible e.g. the 'snap' function when joining timbers doesn't seem as intuitive as I'd like.

Just done the base so far, converting the hand drawn sketch above onto the app. Foundations next.

garden room v5.png
your issue for planning doesn’t need an architect - they will be happy with a pencil sketch and location plan downloaded off one of the OS map providers. Your issue is size and you are in building regs territory over 30sqm so I would seriously consider reducing the size / length slightly which will mean you don’t need approvals on structure / insulation / etc.

You could retain the total length as designed but add an open / covered area to one end so you would have an 8.4m internal space with a 1.2m open end - this wouldn’t need BRegs approval but would be just as useful.

Also consider your roof design - a standard 22.5° apex roof will need around 8m of timber per truss profile, a 5° mono pitch will use half that. The apex roof will also require around 8-10% more covering, double the length of guttering and so on.

If you want to get a lot of light into the shop but retain some security and wall space then also using a celestory or high level window would also benefit - using your panel design to create a rectangular box, you can then add on the top lights all along one edge which will increase light into the build but also mean you maximise wall space internally.

I'm running with this idea and trying the building slightly shorter, at 8.4m x 3.6m. This leaves 1.2m of covered decking. I'm also planning to run the raised decking around the front too but not including this on the plans yet.

Since the 1.2m will be covered by the roof, I'm including this within the base design, as the roof above will need proper support. The original layout of the concrete blocks was an even spacing but I've added an extra column of blocks sitting underneath the new corner at 1.2m.

I'm not sure of the best layout tbh... (for info - I'm also going to develop this design with a pent roof)

garden room v5 (1).png

garden room v5 (2).png
For the sake of a few blocks, I think spanning 2.4m between piers is going to be pushing it especially with equipment etc loadings on the floor. I would go with 1.2m spans assuming these are single block on flat piers.
The blocks I have planned are the large high density hollow blocks. I have quite a few remaining from a professionally built retaining wall. They measure approx 440 x 220 x 220. My plan is to place two, side by side, and fill with rebar & concrete, driving the rebar down into the rocky ground below. I’d estimate I could achieve between 0.5/1m of depth for the rebar.

The blocks will sit on a 880 x 880 size sub base of either type 1 or concrete, to a depth of approx 300mm. Not sure what’s best, compacted type 1 or concrete.

I’m reluctant to go as substantial for any intermediate blocks. Feels overkill. How about some standard 7.3N ones belly down, for the 1.2m supports in between my main piers? Is there any risk when mixing different size piers?
If I’ve got my calcs right, then the spans between piers are currently:

2400 - 220 - 220 = 1960

Which I think is ok for 6x2 at 600mm centres. But I take your point…
I’d just use them as they stand individually - won’t go anywhere if they are sat on a concrete pad and the rebar is probably only going to stop them moving sideways so 500mm tops will work.
Thanks both. Currently working on a version that sticks with the main piers as shown (440 x 440) every 2.4m but adds single blocks (220 x 440) at the 1.2m points. Bit of a hybrid but I’ve got married to the idea of the main piers being 2 blocks!
Some random points.

I've not used the latest version of Sketchup, so I can't be sure, but my prejudice says that an online tool accessed via a web browser (?) is going to be clunkier than a local app. There are sites which have archived old versions of free software - see if you can track down Sketchup Make 2017?

Remember that Building Regs threshold is internal floor area, not external building footprint. That said, obviously you want to avoid paying the oh-so-reasonable fees, but actually complying with the regs oughtn't to be a problem, unless you're building a ramshackle fally-down deathtrap of a shack. Which you're not.

Part P applies whatever though, but the electrician for the main work will be able to take care of that. Do you know what size the existing cable is? Better to be too big at first than too small in a few years time if your use changes and you need to trench a bigger one in.

Ditto other services, if they won't give planning a hissy fit (hint - don't tell them about water and drainage if you aren't proposing to use/connect them in this shed)

I don't know if Article 2(3) makes a difference, but if you build it so that it is capable of being moved as a complete structure (even if access realities mean you'd need a Sikorsky to do it) you may be able to avoid PP as it will count as a caravan.

And as for access - you can get micro diggers which are only 69cm wide.

Consider an ASHP for heating. (And cooling ;))
Instead of blocks what I have used in the past is concrete fence posts, my office shed has been there for about twenty five years without moving and its on a slope.
Anyone seen these insulated panels before? I’m not really interested for the main structure but they sell them as roof panels only to be fixed onto timber frame buildings. Looks neat.

Anyone seen these insulated panels before? I’m not really interested for the main structure but they sell them as roof panels only to be fixed onto timber frame buildings. Looks neat.

Anyone seen these insulated panels before? I’m not really interested for the main structure but they sell them as roof panels only to be fixed onto timber frame buildings. Looks neat.

They are an off shoot from Kingspan from what I can see. I've been looking at one of the buildings for my homebrew setup. Basically, it is Kingspan insulation sandwiched between 2 x 0.5mm sheets of treated steel. They reckon it is impossible for the steel to rust due to the process it goes through.
Hope this helps.

I assume that counts as substantially non-combustible, given how close it is to the boundary. Talking of which, I don't know whose fence that is, but if it's his he'll regret not replacing it while he could still get at it.

As for speed - I recently had a 3m x 11m log cabin built in about 1½ days.
Hi all

I'm starting this thread to document my work on this new garden room build and hopefully get some pointers from far more experienced readers! A couple of times now I've asked questions on other threads, so thought it was time to start my own.

Disclaimer: this project will be very much DIY... I have no building quals and work in a completely unrelated profession so learning all the time. Plus, such as life, my time will be limited so this build is likely to evolve over quite a few months. Planning has already been months... if not years.


Approx 2 years ago I built a small non-insulated shed as a trial run - 2.4m x 2.4m, apex felted roof, featherboard cladding - it went well. I used 4 equal sized framed panels, 16" centres and double top plates binding the wall panels together. I've created a very solid building that I now wish to replicate but on a larger scale.

I'm not short of garden space, so the plan is to build a 9.6m x 3.6m wooden framed building, apex tiled roof and insulated. It will initially need to be a flexible space as we are having some substantial building work done on the main house and need a separate area to escape to when things get really disruptive.


As far as planning goes, I've been in touch with the local planning dept. and they advise that permission will be required as we sit within Article 2(3) land and the building will sit over 20m from the main dwelling. This is actually not an entirely bad thing, as I now feel at liberty to design it to my desired size/height and make the most of the land available. There has also been a precedent set by several similar buildings in neighbouring gardens. A local arch' tech has offered to create the drawings and process the application. Although this comes at a cost, I'll be glad of his support and I will definitely need help with the roof design.

Work process

I'm aiming to do as much of the work as possible myself but not yet ruled out the option of getting some help. e.g. there is already an armoured cable running to the site from the main house, as this provided the previous garden shed with power (installed by the previous owner with electricians certs provided). I'll use an electrician to complete all electrical work. I don't currently aim to have any other services running to/from the building, although I could easily be persuaded otherwise!

There are some tasks that I know will be a stetch e.g. roof tiling, doors and windows. So i can imagine drafting in help for certain parts of the build, if possible.


First off - I'm not getting on well with SketchUp in the slightest! Can't fathom it whatsoever. I have many drawings on paper and will try to upload some once neat enough for public viewing.

Onto the plan... because I lack any great skills or time (plus £ is always an important factor) I am keen to minimise cutting of sheet materials, timber and overall any waste. I notice Robin Clevett mentions the use of 450mm centres for framing, giving 3 equal cuts to 1200mm PIR i.e. 400mm panels of PIR with tape or foam to make up. 25mm each side being taken up by half a stud. He also mentions using 2700mm sheet materials (OSB) to accommodate 6 equal stud spacings. This sounds great in theory, but the 2700 OSB seems much more difficult to source and I'm reluctant to introduce any barriers that prevent obtaining good value timber. So taking this concept a little further, I realise 450mm goes into 3600mm equally by 8. So panels of 3.6m width seems a good solution.

Whilst this takes care neatly of the PIR, there is then the issue of sheet materials being mainly sold imperial (2440 x 1220). However, I figured that if I go for 9mm external OSB sheeting, this is readily available in metric size of 2400 x 1200. I'm likely to use both an external and internal skin of OSB so the smaller thickness seems acceptable? Therefore a wall panel of 3600mm width and 2400mm height would be covered economically by 2 horizontal sheets and 1 vertical i.e. total width of panel = 2400 + 1200, total height of panel = 1200 + 1200 or 2400.

All excellent... except... 3.6m x 2 gives a building of 7.2m - too small, 3.6m x 3 gives a building of 10.8m - just a fraction too big.

So my current plan is to use:
  • Long walls - 2 panels of 3.6m plus 1 panel of 2.4m
  • Short walls - 1 panel of 3.6m
This gives a total footprint of 9.6m x 3.6m.

I will keep to 450mm centres on the 2.4m panels and just reduce the final stud spacing to accommodate.

My current thinking is to build studs using 4x2 timbers. I need help with roof design but would expect 6x2 rafters and ridge beam.

Groundworks and foundations

The area is not easily accessible by vehicle, being quite some distance from the road. The site is on a gentle sloping gradient, falling on the shorter sides by approx. 400mm.

View attachment 136940

The ground is very rocky with the area historically being quarried for limestone. I'm therefore not inclined to use a concrete base and instead use piers of some sort. Following on from some discussion on another thread about suitability of concrete blocks and appropriate depth of footings, I have since dug a couple of trial holes to investigate the ground conditions. The top layer is approx 15-20cm of softer stony ground and below that you hit rock (see first pic). Limestone being the way it is, the rocks are large pieces compressed together around smaller pieces and grit (see second pic) but there is another section of the garden where I've gone deep enough to hit pure limestone strata.

View attachment 136939
View attachment 136941

Given the ground conditions, I don't believe a great depth of footing to be necessary. Instead I plan to excavate the top 200mm of softer ground and use hollow dense concrete blocks sat either on a bed of compacted MOT type 1 OR concrete. I'm thinking 2 blocks for the higher ground piers and 4 stacked blocks for the lower ground. These can potentially be filled with concrete and rebar driven through the hollows into the rock below.

I have quite a few of these blocks left over from a retaining wall that was built (professionally) so it does make sense to put them to use.

Using the general rule of footings being at least twice the width of the block, I propose digging these footings 880mm x 880mm, which would then accommodate two blocks side by side, measuring 440mm x 440mm.

I have no idea how deep I'd get the rebar... the ground below the 200mm is seriously hard.. and I don't know how much use the rebar would be? I could also tie together the blocks together using some sort of metal ties, although I think the pre-cast grooves are on the shorter ends not width, so don't know how that would work.

Next steps

Most importantly, I need to get the plans drawn up and submitted to the planning dept.

If I feel confident enough, in the meantime I'll put some hand drawn ones on here so anyone can tell me I'm way off track or doing ok.

Block piers ... I'm seriously quite stuck on the spacing of these. I've looked at the span tables and if 6x2 C16 timber is used for bearers and floor joists at 450mm centres, then I think spans of 3006mm are permissible for buildings of less than 0.25kN/m2. However, I notice on Molynoox's build (which is a similar size of 9m x 3.7m) the groundscrew company were wanting spans of 1.5m but this was doubled up 4x2... my head is in a bit of a spin about how to go about designing this raft and piers... any help would be massively appreciated. I'm guessing some idea of the loading would be helpful! I'm very much hoping for fibre cement tiles which I know adds weight...

Final request - please don't tell me to go out and buy a ready-made. I just can't stomach them and if this takes me many, many years, then so be it!
I built something similar 16 years ago, though not quite as impressive size wise. I used 3x2 framing and Shiplap with glass breathable batts for insulation. I laid the whole thing on a concrete pad with damp proof membrane. The frame sits 5mm neyond the outer edge of the concrete pad. The bottom Shiplap board extends an inch below the top surface down the outside of the pad. To date this still looks as good as new and is Snuff-dry inside.

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