As far as I am aware building regs state that there has to be 6' 6" ( 2metres ) from any tread to the lowest part of the ceiling. I considered doing a similar thing but by creating a built in wardrobe with a raised floor.
If it's just a pipe that has been buried but not connected to a water supply then you will need to dig a trench across the front of the out building approx 600- 800mm deep.
Knock or cut holes/ slots into the exsisting wall as you build up the extension cavity wal and build your concrete blocks and new slate into the outer skin of the slate of the old wall then reinstate the holes/slots to tie the new wall into the old one.. Do that several times as your extension...
The only sure way of preventing freezing is to turn off water at the main stopcock. Open all taps and leave them open. drain down central heasting boiler and any hot water and header tanks.
Download the manual here and see if thart tells you what the problem might be https://www.worcester-bosch.co.uk/professional/support/literature/worcester-si-ii-installation-instructions
You say you could use a ramp with difficulty. Would it be possible to use a longer ramp to reduce the angle and to fit a small winch ( as fitted to cars ) to pull your father up the ramp to the house or to the wheel chair itself...
Is ther a hook sticking out of the front possibly on the LHS. If so it is probably part of this as is the switch you are looking for
https://www.365varaosat.fi/cooker-oven/c7gpx-si00/door-lock-assembly/product.pl?pid=2085994&shop=smeg&path=110769&refine=door&model_ref=15193486
You can see it's...