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Question about extending a "Compton" type sectional concrete garage. - Now with 100% extra buildings control question, FREE!

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Jelly

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TL;DR:
Has anyone got experience of extending one of the "Compton" type sectional concrete garages?

(i.e this type, although this isn't mine, just an example I found online)

  • Are they capable of meeting the building regs if they go over the "magic" 30m² exemption?
  • Is it possible to make an exterior internal corner with the panels (see option 2 on diagram below)?
  • I'm Quite concerned about the structural stability of the garage when taking out the eaves and rear panels out in order to add length; should I be?
  • I'd like to fit "window" half panels to replace existing full height ones, and i'm very concerned about the structural stability of the garage whilst I'm doing this; again should I be?




Detailed explanatory aside:
I've recently acquired a house with a good sized (3.5m × 7.4m) concrete garage, with the plan of setting up my metal-bashing tools (Mill, Little Lathe, Big Lathe, Surface Plate, Drill, Grinders, Welder, Pipe Threader etc) in there.

However whilst i was preparing to insulate and board it, I realised that it also needed a new garage door to give me a cat in hell's chance of making it dry and warm enough to avoid serious rust issues once I clean the packing grease off my stored tools.

I used to make doors for a living once upon a time, so after a bit of research into bifold door mechanisms I drew some plans and bought 0.3m³ of unsorted redwood with the intention of making a bifold garage door, However upon taking delivery of the redwood I realised that all the rebates and tongue and groove profiles I had planned to plane by hand, mounted up to an awful lot of timber to remove...

To recuperate from this terrible realisation, I availed myself of a number of pear-derived refreshing beverages (actual Perry, not the alcopop in all but name stuff) which unaccountably turn out to be strongly alcoholic... With my judgement slightly impaired, I ended up idly browsing a well known internet auction site, and subsequently purchasing a nice little spindle moulder for a really very good price (so good in fact, that the spindle moulder and the wood are still less than buying the doors commercially).

There's not the space for seperate woodworking and metalworking workshops in the garage, and after spending nearly 2 years doing a nut and bolt rebuild of the mill (including a whole new spindle), I'm not willing to have wood dust in the same room as the machine tools. So, after making the doors, and before i finish setting up the workshop it would have to go, which seems silly as I'd never get another for that kind of price; and seen as I also have my little ELU Planer-Thicknesser on long term loan to someone whilst they refurbish a big Wadkin planer thicknesser, and a Radial Arm Saw which has found a temporary home (and occasional use) with another friend, it would be really nice to be able to get those back, and set up a decent woodworking shop too...

As it happens, there's an additional 4 to 4.5m (ish) of space to the rear of the garage which is basically dead space (and is full of dead and dying Leylandii) that could become garage, and an additional 2.5 × 2m area which is currently infested with bamboo and also wouldn't be missed; if I was to add these additional areas I'd have plenty of room for dedicated wood and metal workshops which were internally divided, could be absolutely sure about the slab-thickness and ground-loading capacity of the back section when I bring the big lathe in, and would have a little annex to do welding and grinding in, to protect the machines from abrasive dust and keep the sparks far from the woodworking area.

If I'm doing this, I'd be keen to save a bit of money on the paneling by only purchasing the cheaper half-height "window" panels, then re-using existing full-height panels I took out. Moreover, if I bulk buy the timber again, the frames will work out cheaper (excluding my labour) than the additional cost for full height panels, which would help to offset the cost of buying the sealed DG-units to go into said windows (which again if I'm ordering 8-10 in a basic spec, shouldn't be exorbitant).

Extention.png


Even better this is all well within my permitted development rights, as the land parcel associated with the house is substantial (even if a lot of it is not strictly my garden, but shared with my neighbours under an easement).


However there are three problems:

  • Over 30m² of floor-space (I already have 26m² in the garage, and another 3.5m² of other outbuildings, so I can't avoid going over) I would no longer be exempt from building control, and I'm:
    1. Not at all sure that a sectional garage can meet the building regs.
    2. Really struggling to find out which parts would apply, so I can research to understand the answer to question 1
    3. If the area of the other outbuildings is considered, I will start to run quite close to the 50m² limit at which the energy efficiency requirements of the building regs will also kick in.

  • The way these compton garage shells are constructed seems a bit sketchy at best, and I'm not clear if it will respond well to me:
    1. Removing the back wall and eaves in order to add additional length
    2. Removing full height panels mid-way along existing walls, to replace them with half-height panels (which are cheaper) to fit additional windows, which is a prefered option to add more natural light.

  • I'm not sure you can have exterior internal corners using the compton-type panels, which might rule out my most prefered option (2, which is L-shaped to give sufficient separation from a mature tree I don't want to fell).


Edit:
Yes, I know it sounds kind of insane to commit to this style of building, but it is substantially cheaper and more convenient than rebuilding in brick; and the money saved there would help finance getting a 3-phase connection (conveniently, I have 3-phase to the meter, so forking that to add a connection on a separate meter in the workshop is actually pretty "cheap", at least in terms of 3-phase connection costs).
 
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MikeG.

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Very hard to comment without seeing photos etc, but I would always say in these circumstances to build a separate building. That way all the questions about stability of the existing and BR compliance of the total build area becomes moot. I've a friend who needed a quadruple garage, and I designed him two double garages instead, side by side. It saved an immense amount of trouble and money. So in your circumstances I would build an in-line new building say 1500 away from the existing, with a door directly opposite the one in the back of the existing, and put a small roof over the gap between the two (just wider than the doors. Make sure this outside area isn't cast concrete, but is paving slabs.

That way you aren't tied to the second-best construction technique of those pre-cast sectional buildings, and can build yourself a proper building (see my signature). Your second building has the same 30 sq m max floor area for BR avoidance. And if there isn't anything in the way, there is no need to be restricted to the width of the existing building. You could do a squarer one.
 

Jelly

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I would always say in these circumstances to build a separate building.
I hadn't really thought of that, but it's probably actually a better idea, in an Ideal world I'd like to maintain close control over temperature and humidity (16-22°C and <45% RH) in the metal side of the workshop, so a dedicated structure would allow me to do that more efficiently (and add the environmental control systems required without a monstrous cludge) all with much less faff.

To make the best of it I would probably end up still shortening the existing building if I did that to get the right balance of area between the two which does leave me still wrestling with the stability issues of modifying the sectional structure.


Put a small roof over the gap between the two (just wider than the doors).

Make sure this outside area isn't cast concrete, but is paving slabs.
I assume this advice is to avoid it being treated as a single contagious structure by a building inspector, should they ever visit.

If not, it there a practical reason for these pieces of advice?

On the concrete in particular it would be practically advantageous to have a single slab cast at the level of the original one, and abutting it, as some of the equipment I may end up moving in there will be extremely heavy (probably max 3000kg, but still that's a big hunk of cast iron).



That way you aren't tied to the second-best construction technique of those pre-cast sectional buildings, and can build yourself a proper building (see my signature).
I've had a look at the design and really like it... However I'm too close to the boundary on both sides to use timber-clad.

I suppose there's cementacious board or steel as options, of the two I think I'd lean towards steel, but is there an advantage to the cementatious board and retaining the shiplap design?


And if there isn't anything in the way, there is no need to be restricted to the width of the existing building. You could do a squarer one.
I would dearly love to, but I'm restricted by the width of the land I own (whilst my plot is only about 3.8 m wide, it's over 100m long) and by dealing with mature trees I don't want to fell.
 

MikeG.

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.....I assume this advice is to avoid it being treated as a single contagious structure by a building inspector, should they ever visit.

If not, it there a practical reason for these pieces of advice?
Yes. They must be two distinct buildings, leaving no opportunity to argue that it's one building divided into two.

On the concrete in particular it would be practically advantageous to have a single slab cast at the level of the original one, and abutting it, as some of the equipment I may end up moving in there will be extremely heavy (probably max 3000kg, but still that's a big hunk of cast iron).
Fair enough. If, once your machinery is in place, if a building inspector did happen to get involved you could always offer to break up the bit of external concrete between the two. He'd never bother taking you up on it, but it would make the point.


I've had a look at the design and really like it... However I'm too close to the boundary on both sides to use timber-clad.

I suppose there's cementacious board or steel as options, of the two I think I'd lean towards steel, but is there an advantage to the cementatious board and retaining the shiplap design?
Or render. You just have to be careful with your ventilation behind it (airflow top and bottom).
 

AJB Temple

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These pre-fab buildings are beyond ugly. Sorry to anyone who likes them. At the very least render the existing as Mike suggests, and paint it.
 

Jelly

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Or render. You just have to be careful with your ventilation behind it (airflow top and bottom).
As in lime-render (or it's Portland based equivalent) over a suitably durable timber substrate, a bit like an external version of Lath and Plaster? Again I didn't even realize this was an option.

I can see that looking very attractive even compared to wood actually, now I think about it...

Thanks Mike, you've given me plenty of food for thought now.


"single contagious structure"

Concrete cancer? Just sayin'...
My phone apparently refuses to acknowledge the existence of the word "Contiguous".
 
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