Options for building extension on existing outbuilding foundation?

[dwtowner]

I have a garage-sized outbuilding behind my house for which I have PP to convert into a room, built using SIPs. The existing garage has been there for about 70 years and doesn't have any cracks or issues that I can see with its foundations. The attached diagram shows the existing foundation in orange, relative to the house and surrounding ground. The other attached drawing shows the footprint of the new building in red. The new building will be about 40cm higher than the existing foundation. There is a mature self-seeded Walnut tree about 1.2m from the foundations which our neighbours are very reluctant to remove (technically it is mine so I could, but I quite like it there too). The edge of the left-hand green strip in the diagram is the boundary with my neighbour. There is limited access - about 1.2m width - for heavy machinery to get to the site. I'd like to understand what the options might be for extending or replacing the foundation to make it suitable for the new SIPs room.

 

1) Rip out the old foundations and pour new foundations. Clean slate to start with, but requires the tree to go, and is very disruptive for the neighbours during the work (their house is only a few metres away). Difficult to get machinery in and spoil out.

2) Leave the existing foundation but use ground screws across the site (through holes drilled in the old foundation) and around the perimeter of the new building, and then put a wooden insulated structure on top to support the SIPs. The tree could stay, and there is less disruption from removing the old slab. 

3) Could the old slab be reused, with some new structure built partially on top of it? On the right of the diagram I think I would need to ground screw or do conventional foundation to support the edge, but could I cantilever off the left-hand side to avoid having to do anything else near by neighbour, and to avoid removing the tree? I've got about 40cm height to play with

4) Something I haven't thought of?

 

Finally, any recommendations for a SE who could help with this, in the Bath/Trowbridge area?

 

thanks,

 

dan. 

 

 

[Nickfromwales]

Hi.

 

Why go for the expense of SIP's when you could just stick build this with a local builder for less money and far more flexibility? That just seems way OTT imho. Very easy to get this to a similar, or better standard, without the bespoke costs and difficulty in joining these things up / together. There's very little point getting this to a much higher standard then the building it is being joined to, so you may want to rethink and save some money and grief here. A local builder would fly this up, even timber frame with a brick façade, and they'd need very little input from a designer.

 

Does the 40cm mean you would have to step up into this extension, or would that still leave you with a level threshold internally?

 

Any new works that close to the tree will anger the rules & regulations gods, to SE will prob say rip out the existing and start afresh, which is absolutely what I would do vs throwing good money after bad. FWIW I would avoid a ventilated wooden suspended floor like the plague, and instead go for a nice, solid, well-insulated slab

[dwtowner]

Thanks for your comments Nick. Some useful prompts to think about things again. A few responses.

 

This will be the third SIPs extension to my house, which itself is very insulated, triple glazed, etc., so I think it is in proportion. I've had a very positive experience with SIPs so far - a kit of parts arrives, all nicely numbered, and up it goes in a day or so. I take your point though that maybe its overkill for this slightly smaller extension.

 

The old garage was lower than the existing house so the extra 40cm brings the floor to the same level.

 

thanks,

 

dan

 

[nod]

It’s much easier and safer to put new foundations in I can’t imagine a Structual engineer putting his or her name to this They normally play it safe 

In any case BC may come along and simply tell you to dig them out