My last shed had to be extended when I bought a milling machine. It was 12' by 8' and I went for a 4' extention but because of its location this had to be done by extending each end by 2', oh yes!, the thing was built on a sloping site of about 12". So I dug out foundations at one end, built a retaining wall, back filled it and put a 4" concrete top to it. My plan A, was then to roll the shed over this new "base" then rebuild it at the other end. This I thougth I could do by putting scaffold poles under the shed and use them as rollers . The plan failed!!, there was not enough clearance under the shed for the scaffold poles and trying to slide them underneath caused the poles to jam and scar the underneath of the shed's floor. So on to plan B
Foundations were dug and the base extended at the other end. Both end walls of the shed were then taken down, leaving the end trusses with their vertical struts under their centres. One end was then extended and re-clad using new shiplac boarding, re-using the boards from end to "fill in " the 2' flont and back, so it matched the rest of the front..
At the othe end the roof truss was re-inforced with steel plate, so the roof weight would be taken down to the side walls and the vertical strut removed , and the end wall wall was built from scratch. Again the front and back wall "extensions" were built using the old planking fom the end. I moved the door from the this end round to the front with a short panel about 12" between tit and the corner, this was so I could unscrwew the door frame and this panel, to give me a 4' width to get the new machine .
So I had a clear area at one end for the mill and a 2' wide extra bit with a roof pillar between it and the rest of the shed at the other end. I used this area as a "store" (biggest junk box in the world?)
Eventually i settled a a "Deckel"stlye mill (low head height precluded a "Bridgeport"
, which was brought home by a Hiab equipped vehicle, which had to lift it over a 8' Victorian brick wall to get it into my garden – wish I had taken some photographs of it.
Frank