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The terms of your lease won't magically change when the freeholder, by proxy, adds the extension. You'll still be responsible for what your lease defines.
Which is probably shared cost of maintenance of the roof.
This makes you a stakeholder in your neighbour's extension building as Aggi has highlighted, but because you aren't SOF then your control in this situation is limited to statutory and whatever is allowed by your lease.
What kind of property? How many more leaseholders? If I were you I'd try to buy the freehold out before the work kicks off then block the work until the leases can be sorted more in your favour.
We’re leaseholders so they would have had to get permission from out freeholders. Just wondering what the deal is in terms of who’s responsible going forward in these cases.