Extension on top of single brick garage.

[mattu27]

Hello,

Had the idea of adding an extension onto the side of the house for a while and have only just decided to put some serious thought into it.

We bought the house in 2014 and have got the plans for the garage (joined to house) which was built circa 1994. The plans show that the foundations were done via trench fill and are 600mm wide x 1000m deep. The garage is built single skin but has supporting brick columns every 2.2m along the length of the wall. So in essence two brick with no cavity at those points.

We had building control out six months ago just to ask some questions and they advised that provided the columns are built off the footings, we would be able to use steels on top of these to build the inner skin off providing that structural calcs are provided. This would then mean insulating the internal walls to get the right U values.

I've been out and dug a test pit this morning and have found that we have two bricks below ground level and then the concrete depth is 650mm beneath this onto solid red clay, 850 ish all in. Width was 700mm

Where do I stand with this? Could it be that they are just shallower at the end of the trench? If building control verified that they were 1m deep in 1994 doesn't this decision still stand?

Going to give them a call and see what they say but wanted to ask the real experts on here before I progress any further. I don't fancy underpinning as it wouldn't be worth it, we're talking 14m worth and cutting the garage floor up if so.

Thanks, Matt

 

[Bonni]

If you have started work back then, then you would only need to comply to the regs at that time. If you're applying to extend now and applying for building regs, then you need to comply to current regs.

All you can do is get a structural engineer to draw up what's needed, showing him/her the test hole you've dug.

 

[mattu27]

Thanks for the reply Bonni,

Wouldn't the regs have been 1m deep back in 1994 for footings? I can't imagine they'd have got it passed off if they hadn't done a metre as the plans state.

The last building control guy said it could be done on permitted development but did mention the digging of a test pit to confirm.

So I just ring and explain the situation and see what happens?

Thanks

 

[^woody^]

If building control verified that they were 1m deep in 1994 doesn't this decision still stand?

All existing foundations will tend to need exposing and checking. Even if they where done last year.

The regulations don't specify a depth. The requirement is that the foundations are suitable. Ground and situations change, so must foundation depths.

 

[lt8480]

I would take whatever they visually appear to be and what they are measured as to be their actual size (rather than what BC may have deemed them to be in 1994). Obviously the design should work for the actual size of footing that exists.

Putting steels on top of the walls sounds technically possible but a bit mad. It's probably cheaper, simpler and better to knock it down and start again, or, why not simply construct an inner block wall to form a cavity wall?

 

[freddiemercurystwin]

I would take whatever they visually appear to be and what they are measured as to be their actual size (rather than what BC may have deemed them to be in 1994). Obviously the design should work for the actual size of footing that exists.

Putting steels on top of the walls sounds technically possible but a bit mad. It's probably cheaper, simpler and better to knock it down and start again, or, why not simply construct an inner block wall to form a cavity wall?

Steels on top of the brick piers is a pretty standard way to add a room above a garage of this kind of construction. Can as easily be done with say 3 timber joists sandwiched together to act as the lintel instead to build the inner skin/floor off. Normally a timber inner skin and brick or block outer skin. A big house developer circa 1994 would typically have just done the same trench width/depth for every wall under the house including the garage, quicker than faffing around digging one a bit smaller to suit a single skin.

 

[lt8480]

Steels on top of the brick piers is a pretty standard way to add a room above a garage of this kind of construction. Can as easily be done with say 3 timber joists sandwiched together to act as the lintel instead to build the inner skin/floor off. Normally a timber inner skin and brick or block outer skin. A big house developer circa 1994 would typically have just done the same trench width/depth for every wall under the house including the garage, quicker than faffing around digging one a bit smaller to suit a single skin.

If the garage is already converted it might make sense to retain and steel over. But if its just a single skin brick wall garage I'd still suggest exploring rebuilding. If space permits you could also then widen slightly for minimal cost and make the bedroom above a double. You can also build in a way that would make converting the garage into a room later an option for almost no cost too.

 

[freddiemercurystwin]

How can knocking down a garage and rebuilding it possibly be cheaper than adding some beams to the top of the existing piers?

 

[mattu27]

Thanks for the replies, just to give a bit more information, the house is a 1953 build and the garage was added on in 94.
It is about 13m x 4m with full electrics so I don't want to pull it down and start again. Plus it's full to the brim of clutter! And would cost a small fortune for temporary storage.

I'm open to cutting the garage floor and removing a section around the perimeter in order to start a second skin off the foundations in bloclwork if that would prove less expensive that having structural calcs and putting steels in above the piers.

But the timber inner skin could be an option if it comes to it, would one just insulate between the studwork as normal in that case?

Building control are coming round tomorrow so I'll keep you posted.

 

[freddiemercurystwin]

OH OK, so check the found sizes and go from there. Steels or timber beams on the piers is still likely the way to go, cost wise and timber inner skin to keep the weight to a minimum is also preferable, there's no problem building with this method. And if you have any disillusions about not having to completely empty it whilst the work goes ahead forget it right now.

 

[lt8480]

How can knocking down a garage and rebuilding it possibly be cheaper than adding some beams to the top of the existing piers?

Sometimes it is cheaper and often done, really depends what exists there now and what is proposed and working out the additional things that may be required due to retention. E.g. depending on existing/proposed may need calculations, additional steelwork, lifting equipment, pad stones / plates, adjusting existing structure, less conventional construction above, underpinning etc.

Its likely to be cheaper if underpinning is required (plus many insurers add a small premium if underpinning work done so that's an ongoing cost), or if a couple of steels are required for retention these are possible more expensive to buy and install than rebuild cost, etc.

 

[mattu27]

Update,

Eventually got building control to come out this morning, test pit was checked and he was happy with the depth of the footings.

Suggestions were:

1) To put steels on top of the existing piers and insulate the inner skin,

2) Dig around the piers and add additional brickwork to support the piers and gain a cavity

3) Or dig the floor around the existing brickwork and add the inner skin off the foundations.

I suggested the timber inner skin idea above the steels and he also seemed happy with that but said to email planning and see what they say.

The plan is to remove the additional access door next to the garage door and make the new garage door wider, one of the test pits in below the access door and I found the soil pipe and the water mains in.

In order to put new foundations in this section, am I right in thinking that the concrete is poured around the soil pipe and water mains which will have say 2" of compressible material and a concrete lintel above it to protect them from movement and damage?

The water mains in looks to be galvanised steel, should this be replaced now or left alone? I have the feeling that it's leaking somewhere.

Thanks, Matt

 

[mattu27]

Any help on this would be appreciated. Thanks

 

[lt8480]

Planning won't care how you build it as long as it looks the same as what you proposed.

If the foundations are okay, why do you need new foundations? But yes as you say around pipes will need compressible material, otherwise any movement will crack the pipe. Design/detail will need to be done by a structural engineer, and/or agreed with Building Control.

If the supply is easily accessible it's ideal to replace it now - if its leaking then definitely replace it.

 

[mattu27]

Thanks for the reply, it was more a question as to which is the best financially viable method to build the extension. For the cost of calculations and steelwork I might as well cut the garage floor and do the inner skin in blockwork, but was unsure.?

The only new foundations going in are in the section of the garage where the single door is (shown in photo) as the plan is to remove this and fit a centralised garage door in the garage. I was thinking 50mm celotex cut so that it forms a cylinder around the pipe, if that makes sense.

With regards to the supply, I was going to leave a 40mm waste pipe sat diagonally a within the footings and use this to put the new MDPE pipe through at a later date as one continuous length.

Does all this seem ok?

Thanks Matt