Best approach for building under a floating bay window

[ZeroTools]

Afternoon all!

I have a small office at the front of my house with a floating bay window, which I'd like to knock through to create a 'proper' bay so that I can create a desk space in the resulting recess. Having read up on a few other forum posts and spoken to some builders I have identified 2 different approaches:

1. Knock through, excavate below the bay and pour a proper slab foundation. Tie the new foundation into the existing and build a new cavity wall under the bay, toothed into the existing brickwork, up to the existing window sill line. DPC, cavity and slab insulation installed along the way, standard plasterboard on the inside.

2. Knock through, excavate enough to pour a 'lite' slab, then build a timber framed wall up to the existing window sill line. Insulate with at least 100mm PIR board insulation, clad the outside in brick slips, board the inside with vapour board.

Option 2 seems to be the preferred option as the bay is only just wide enough for a reasonably sized single desk as it stands, so building a cavity wall would leave very little space if keeping the existing window footprint (which I want to do as the frames are in good order). But I want to make sure I don't end up with cold and damp feet when sat at my desk!

Some pictures attached for context - opinions and considerations on the best approach would be welcome!

 

[^woody^]

Oriel window.

You'll need planning and building regulation approval, as it will be an extension.

You should get it properly designed by a suitably qualified/ experienced designer and not rely on the whims of builders who would love to build their own biased/poor/wrong/ recommendations - which typically benefit them not the customer.