Thank you for your reply.
According to your two points, this is what the lease says:
"Not at any time during the said term without the licence in writing of the lessor first obtained to erect or place and additional building or erection of any part of the demised premises other than a shed for domestic purposes only and not without such licence as aforesaid to make any alteration in the plan or elevation of the maisonette hereby demised or in any of the party walls or the principal or bearing walls or timbers thereof nor construct any gateway or opening in any of the fences bounding the demised premises".
Is this applicable to loft conversion?
What do you refer to as the plan is altered? is this the plan of the original house that is now divided into two flats? If so, a roof expansion will certainly alter it. Please see below.
In the 2nd clause there is a mention to no alteration in "timber". The roof frame is timber and a loft conversion will certainly need to alter it to create more space, including windows through the roof, and if he decides to create and a bathroom, that is even worst. Can alteration in timber be referred to the roof structure?