Can you cut into the Timber behind the Door Lining?

You will need to install a proper lintel, because I can see that blockwork dropping, also the weight of the block work will cause the head of the door frame sag, so doors will stick, drag.

What the width of the doorway.

Either a prestressed concrete lintel, might even get away with a metal corrugated lintel, but that depends on the width of doorway & overall loading.

 

Harps, you need someone on site who knows what they are doing - to check if that wall is supporting anything in the floor above.

If that wall is NOT a load-bearing wall - and I hope it isn't, since it has no proper lintel - then you could remove the concrete blocks above the door and move that top frame rail upwards.

But do not do that unless you are 100% certain it isn't load-bearing.

Anyways, the usual thing to do is to trim down the new door to fit...

 

You can trim pretty much any 35mm door by 30mm.

If it is a hollow panel door you may find that you have removed most or all of the bottom frame member in which case you just make a new one and tap it in using a good quality glue to hold everything together. Put clamps on until the glue is set.

If you take 15mm off top and bottom you should still have some frame left but may well hit the pins that they use to hold the frame together whilst making the door. Just be prepared for a lot of sparks of your saw blade (eye protection) and cut slowly.

If it's a solid door there's nothing to stop you taking a lot off.

If you read the blurb that comes with most doors you can't do anything without invalidating the warranty - that's just to protect the manufacturer but you can still do it. Just at your risk.

Changing the door liner seems like a case of 'tail wagging the dog' and would certainly be a lot more work / mess and might have real risks associated with it.

 

DAs answer is the way to go and a whole lot easier than cutting to door down then fitting a new infill to it.

 

It looks like a standard bit of build, where the section above the door is fitted onto the timber noggin. It may be blocks or bricks and plaster that can be removed and you can put in the old noggin or a new one higher up. I have done this on a whole house following a loft conversion, where all the fire doors had to remain as they were manufactured. ie no trimmings.