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Hi Thank you for your question and welcome my name is ***** ***** I will assist you. The landlord wont be able to forfeit the lease for simply putting in double glazing without permission - forfeiture is a likely remedy if you have gone bankrupt for example or you altered the structure and internal layout of the property to the point that it makes the building unsafe. Really you have the following options:
1) You delay completion to go and get permission
2) You allow the buyer to get permission, and agree a retention for the cost of replacing the windows so they match the other residents;
3) You can get indemnity insurance against a breach of lease, but only if you have not sought the permission of the landlord first.
In relation to the electrical works, either say to the buyer either he sorts it himself or go and get an electrician to certify the work. While buyers ask lots of questions, the questions you have described are fairly common and probably wont put off a serious buyer- in all likelihood he is trying look for reasons to drop the price.
Kind regards AJ