< snort > at the stately home , it's far from it . We just have a bit more space as we live in the country .
We had the same problem with our conservatory , which we put in 2.5 years ago ( you can just see the corner in the photo ) . The floor level is now 0.6 m above the ground next to it . It was about a meter Higher but we moved around a lot of dirt to make it more level.
Out extension is at the same floor level as the rest of the house, but I think your plan to drop yours down is excellent . So you'll need to think what to do with where these two different levels join the garden .
As it's North facing, you won't sit there anyway, so why not go for planting beds next to the house, with a retaining wall ? if it's a 1920s house, your windows will be reasonably high so you can grow lovely plants there without blocking your view .
If you are going to have big machinery in your garden anyway, you might as well go the whole hog and do some levelling at the same time . You will be in tears and your whole garden will be a sea of mud , whatever happens .
It's better to spend a few grand extra on ground works now, and save the money elsewhere by putting a scabby old sofa in the lovely new extension . It's easy enough to buy a new sofa in a few years time , when you have the extra cash . But you will never have the chance again to sort out the levels in the garden .
We access the garden by a terrace / set of steps across the whole width of the conservatory . The top " step " had to be quite deep and at the same level as the floor as the doors are folding and they go out the way ( don't know how to describe this better ) . So if the steps went straight down, when the doors are open they would be left ( visually ) hanging in mid air .
We also thought about just having the steps only as wide as the doors, and raised beds with plants all the rest of the way around . But we had enough space to make the steps wider to balance the building visually . If I had a smaller garden , I wouldn't want to waste valuable planting space on boring hard landscaping .