Building a second floor - is underpinning required

[Malli]

Hi

Thanks in advance for taking the time to read.

About our property
A significant portion of the property is single storied. This must have been the parts where the property was extended over time. The original structure is double storied. As a rough estimate, the first floor square footage is half of the ground floor's

Brief Description of Project
We have received planning permission to extend the property. A summary of the change to building
Ground Floor
- Retain the existing footprint
- Remove some of the internal load bearing walls for a more open plan design. Steel beams will replace the load bearing walls
- Some reconfiguration of rooms

First Floor
- Extend over the downstairs footprint, adding more rooms
- Two steel beams added to support the structure

Roof
- Replace the existing roof to make it one single roof.

Trial Pit findings
- Where there is current double storied structure, the half of the foundation exposed (the other half would be inside the property) is 30cm wide x 25 cm deep
- Where there is current single storied structure, the half of the foundation exposed (the other half would be inside the property) is 30cm wide x 30 cm deep

The structural engineer advises us that we'll require underpinning.

Question
To me, as a layman, where the existing double storey structure is supported by a 30cm x 25cm foundation, it's not clear to why isn't the 30cm x 30cm foundation on the single storied side not sufficient when extending to double storied.

Thanks and regards

 

[blup]

It’s not sufficient for a single story under current regs as I understand it. Worth exploring if it’s cheaper/more practical to demolish the single storey structure and rebuild instead of underpinning,

 

[Malli]

Thank you @blup for taking the time to read and respond.

 

[Murdochcat]

Your local BCO will be the best person to check this when you put your application in and pay your money

 

[cdbe]

Assuming it's 600mm wide x 250-300mm thick it sounds adequate for a strip - maybe the issue is the depth and quality of ground underneath - but it should really be the same for one or two storeys but I suspect it isn't always the case.

 

[securespark]

I don't trust my local BCO.

I asked one guy who came to look at our existing extension whether I could use fibreglass to insulate under the floor. I was told it had to be rigid. It was a pig of a job and massively expensive, being after the massive hike in materials prices.

When a second guy came back to inspect that work being done, he said that he would have allowed fibreglass. He also went on to snoop at our en-suite (which was pre-existing and just being refitted) and asked if it was new. I said no, it was being reno'd. Are you sure?, he asked....

He also said we didn't have any paperwork for the extension (done in 1970, 29 years before we bought the house....) and that was an issue.

Then he went away and discovered we did actually have paperwork.

So, setting aside the "looking for jobs", what I'm saying is, in my case, they weren't all singing from the same hymn sheet, which they should, otherwise you don't know where you are.

 

[woodenmecom]

Hey!

Totally get why you're scratching your head about the foundations. It seems counter-intuitive that the 30x30 for the single-story isn't enough, but the 30x25 on the double-story side is.
Here are a few quick thoughts on why your structural engineer might be recommending underpinning:

New Loads: Adding a whole new floor changes how weight is distributed, and the engineer might be concerned the existing 30x30 foundation won't handle those new, increased loads safely.
Soil: The soil under the single-story part might not be as strong or stable as the soil under the existing double-story section, even if the foundation looks similar.
Current Standards: Building codes and engineering practices evolve. What was acceptable when the original structure was built might not meet today's requirements for a significant extension.
Safety Factor: Engineers often design with a healthy margin of safety. They want to be sure everything is super stable, not just barely adequate.

It's definitely worth having another chat with your structural engineer. Maybe ask them:

"Could you explain simply why the 30x30 foundation on the single-story side isn't sufficient for the new two-story extension, given the existing 30x25 is already supporting two floors?"

"Is it about the soil, the new load distribution, or something else specific to this extension?"

Good luck with the project!

 

[yorksbuilder]

Thanks, ChatGPT