Can you build over a lower storey in this way?

[smp50]

Below are four simplified floor plans, black is an existing single storey structure with a flat roof, say, and red is a second storey addition to be built on top (and in #2, #3 and #4 also enlarging the footprint so assume that some extra ground floor is built as well)

I'm pretty sure that #2 and #3 are quite usual and straightforward situations, in #3 you'd need to span that unsupported gap with a steel or something, but you can still do it.

But what about #4? Can one build like that, can it be supported somehow? Let's say the size of the "overlap" is about 1.5m square

 

[^woody^]

Yes cantilevered extensions are relatively easy, or a support post can be used

 

[smp50]

Yes cantilevered extensions are relatively easy, or a support post can be used

There'd be no space for a post in the ground floor room, really. Assuming the same sqm overall, would #4 be more expensive than #3?

 

[^woody^]

For the engineering design and structural support work each would be similar in cost.

 

[smp50]

Sorry to bump this, but I was talking to a builder friend today and he was very keen that #3 be used if at all possible, saying that #4 comes with considerable extra difficulties spanning the roof unless you use a support post inside the ground floor room (which is not possible in this case).

Can I ask for some more comments? I could have the build come out across the whole existing storey but it will be more are and most cost, but if the unusual unsupported corner of #4 is a problem then it may be worth it.

 

[RonnyRaygun]

From what I understand you are not cantilevering the extension, rather building additional ground floor space under the red outline and then building a second storey above.

Neither option #3 or #4 are particularly difficult to construct. Option #3 just needs a single steel beam spanning side to side to pick up the second storey (as you assumed). Option #4 is only marginally more difficult and would require the same steel beam as in option #3, and then an additional beam running perpendicular to the first beam which would support the second storey's other wall.

It really comes down to which option works best for you and which option you will be able to get planning permission for.

 

[smp50]

From what I understand you are not cantilevering the extension, rather building additional ground floor space under the red outline and then building a second storey above.

Neither option #3 or #4 are particularly difficult to construct. Option #3 just needs a single steel beam spanning side to side to pick up the second storey (as you assumed). Option #4 is only marginally more difficult and would require the same steel beam as in option #3, and then an additional beam running perpendicular to the first beam which would support the second storey's other wall.

It really comes down to which option works best for you and which option you will be able to get planning permission for.

Oh right, ok - and excuse my ignorance but would both beams be enclosed in the shallow ceiling/roof space of the existing single-storey part?

 

[RonnyRaygun]

They can be with a little bit of work. It would mean cutting some roof joists and resupporting them off the beam but it's not particularly difficult thing to do.

 

[smp50]

They can be with a little bit of work. It would mean cutting some roof joists and resupporting them off the beam but it's not particularly difficult thing to do.

So that's not the usual way, perhaps? Would they normally be sat atop the roof and then be exposed or kind of boxed in..?