Extending a leasehold flat

[KnottyAuty]

Anyone got advice on this rather niche question? My sister has a 1 bed flat and there’s a large area of common parts right next to it that no one uses. She’s spoken to Planning and they’d be fine if she extended into that area as long as she does a proper application. The leasehold company are interested as if my sister that space from them it will boost funds for repairs (all owners own a share of the freehold company).

So far so good - but apparently the big stumbling block will be the legal costs? All leaseholders and their mortgage companies will have to agree? and all the Land Registry plans have to be adjusted?

there are 20 flats - anyone know how much might this cost her?!

20 flats times changing LReg ownership for each sounds like a steep bill.
Id get her to ask for the cost of doing one first then multiply
Perhaps a solicitor will be on soon to say more accurately
but id make a guess at £20k / £40k

( this is based on the cost of us purchasing a small parcel of unused land with one owner adjacent to ours about ten years ago )

We signed over the shared attic space to our upstairs neighbours for their loft conversion (we didn't charge them but they agreed to redo the whole roof and would take ownership of maintenance). It cost us nothing but cost them around £3k for both sets of legal fees and they also had to pay a surveyor for a new lease plan. With 20 flats to consider, I'd agree that it would be a very steep bill (and a lot of hassle).

Hassle sounds very likely- thanks!
But it might be worth it (just) for a 2nd bedroom