Ground Floor Flat Extension - Steel to support building above

[Will H.]

Hi all, I am in the process of completing on a ground floor flat and wanted to do a rear and side return extension, but wanted to clarify some things before drawing up some plans for planning permission.

The extension I have in mind would require the corner of the building demolished and replaced with two steels to support the rest of the building above. Is this quite risky and expensive ? Just thinking in terms of potential disruption / and damages to the upstairs flats, there are two floors above.

Have anyone had experience with this? It would be good to gauge costs and beam depths as floor to ceiling heights are not great. Alternatively I could build further 1.5m out but would have more planning risk and garden would end up around 6m deep!

Any advice / tips would be hugely appreciated!

Cheers,
Will

 

[bernardgreen]

Is it a Freehold flat or a Leasehold flat ?

If it is Leasehold then you will need the Freeholders permission for that amount of structural changes to what is in reality the Freeholder's property

 

[Will H.]

Yes it’s a leasehold flat, i understand it will require landlord’s consent, planning approval and party wall agreement too.

 

[tony1851]

The main issues which your SE will need to address would be 1. the method of supporting the structure above while the walls are removed and 2. the design of the beams and the connection between them.

The steel beams will deflect under load, and the trick is deciding how much deflection would be tolerable to avoid undue movement upstairs.
There's no hard-and-fast limit on dead load deflection in these circumstances, and it's up to the SEs judgement on how much would be acceptable. All that can be said with certainty is that you would need heavier beams (not necessarily deeper beams) to keep the deflection to a minimum if headroom is an issue.

 

[^woody^]

You would be wise to get the landlords permission first as that's the prime permission above all others, and this work will likely impact on the demise of the leaseholder above and that's another distinct permission required before any other.