Structural question for building over a garage

[Haf63]

I have a 1997 new build with an attached side garage and have got planning for extending the garage backwards to match the rear of the house and add a first floor with a dorma at the front which will be built into existing garage roof. 

 

I am reasonably familiar with extension builds but usually starting from the ground up. In this case I would really like to keep the existing garage wall along the length but its a single skin facing brick wall with a couple of pillars along the 5m length (the front 2m will stay as garage and the extension will add 4m to length). I know that I will have to dig and check foundation but suspect its unlikely to be suitable for building regs. I know that demolish and start again maybe the best option but wanted to explore the options of underpinning and a steel frame. I believe underpinning is pretty expensive versus demolition so was thinking of using some sort of steel frame around the 3m of the existing wall being used so it can support the first floor. A normal foundation can then be dug on the inside for the inner block work and insulation. 

Or am I being a bit nuts and should just knock down the wall where the room starts? 

 

 

[Haf63]

Just a quick update. I dug a test hole to check that foundation and it starts at 800mm below ground and I assume it will be 300mm+ after that. I would hope this is enough to be useable and so keep the existing garage external wall and build on the inside

 

[Big Jimbo]

Keep digging. Find the bottom of the concrete. I'm sure your structural engineer will want that to enable him to do his calculations.

[richo106]

When I exposed the footings on our bungalow so we could check if can add another storey without underpinning. They were 6 to 9 bricks below the blues but the actually concrete footing was only 8” deep and 600mm and this was suitable for a full other storey. I was even that sceptical I paid another SE to run the calculations as well for me

[George]

If the soil is good then width is more relevant than depth. 

 

 

It's clearly got a proper foundation so you will probably be able to reuse it. But get it inspected.

[Haf63]

Thanks for the feedback and I will see if I can dig to see depth of foundation but would hope the 'new build' guys would have at least done the minimum thickness to get me to 1m. 

I wasn't aware of the width being an issue as this looks about 15cm overhang on the outside. The inside is a concrete floor so don't fancy digging that up but there are some blocks pillars on the inside - look like a single blocks mounted on their side against the outer brick wall. If the inside is also 15cm past the block then that would give a width of 500mm. 

Lastly I went to see the neighbour who had something similar done back in 2010  and he said they did absolutely no foundation work at all (didn't even check it) and simply put a steel on top of the block pillars in the garage so the outer wall was a simple continuation of the existing wall and they build the blockwork for the first floor onto of the steel. They also had a steel front and back on existing garage pillars - so it seems like there is an option to use steels to support the first floor regardless of the garage foundations. 

Obviously I am leaving this to the architect and structural engineer to work out but my builder is advising me that his quote would be lower if the existing garage stays mainly intact and he doesn't have to do lots of foundation work hence I am educating myself before I have this discussion with the architect.

 

Appreciate the advise so far. 

[George]

Yeah the external projection is enough as it is a reasonable assumption the wall is placed centrally. So overall foundation width is projection x2 + wall thickness.

 

Your engineer will be able to cross check this with ground conditions to get an allowable line load. It isn't an exact science but without full ground investigation, that is how a lot of domestic foundation work is.

[saveasteading]

As you are extending the building, there is little justification for going any deeper than the existing.

So a few more spade-fulls and you will know.

To avoid digging it again, take a photograph with a tape or measuring stick down to the the bottom of the footing, and showing the depth at ground level. Take a note of the depth from a fixed point: block to brick junction is a good one,  Distance to ground, then to concrete, then of the base itself.

 

150mm outside the wall is pretty standard, so it all looks normal.

Absolutely no need to dig internally.

 

The walls themselves will need to be stiffened... that is a different matter. What is the garage wall construction?

 

And excuse my pedantry. The term is dormer window.

Edited May 10, 2023 by saveasteading

[George]

16 hours ago, saveasteading said:

And excuse my pedantry. The term is dormer window.

 

[Haf63]

16 hours ago, saveasteading said:

The walls themselves will need to be stiffened... that is a different matter. What is the garage wall construction?

 

And excuse my pedantry. The term is dormer window.

 

Appreciate the spelling correction so no excuse needed.

The wall is made of single house facing brick with a few pillars on the inside - not sure what you bean by 'wall stiffening' but sounds sensible

[Haf63]

17 hours ago, George said:

Yeah the external projection is enough as it is a reasonable assumption the wall is placed centrally. So overall foundation width is projection x2 + wall thickness.

 

Your engineer will be able to cross check this with ground conditions to get an allowable line load. It isn't an exact science but without full ground investigation, that is how a lot of domestic foundation work is.

 

Thanks and I am calling in a structural engineer to have a discussion - just trying to track one down

[saveasteading]

4 minutes ago, Haf63 said:

wall stiffening' but sounds sensible

Your house will be either brick and block or timber with a brick face.

The garage is much lighter construction, just holding a roof and no people, so won't support an upper floor without modification. 

SE should suggest the most economical solution.

[George]

14 hours ago, Haf63 said:

 

Thanks and I am calling in a structural engineer to have a discussion - just trying to track one down

What part of the country are you in? 

[Haf63]

On 11/05/2023 at 23:40, George said:

What part of the country are you in? 

I am in warwick (CV35)

[Haf63]

Quick update - structural engineer checkout out my garage and basically said the best route is to build a pillar on the inside and run a steel to the end of the garage which will support the first floor. The existing garage wall and structure looks ok to stay as is. Good news - other than will need to dig a 800x800mm hole inside the garage for the pilar. 

He also suggested I contact the house builders as I purchased from new 25 years ago and they may all be able to give me a copy of the structural documents so will give that a go as well.

[saveasteading]

Remeber you will need insulation and fire protection between the garage and the new room above.