FWIW my brother was in a similar situation.
He bought a flat, one of two, above a run down & vacant carpet shop.The shop was owned by the freeholder. The landlord never contacted my brother for rent /service charges.
Turns out the freeholder had done a flit, having not paid his VAT, and the shop & freehold was put up at auction. Which attracted little interest. My brother and other leaseholder attended auction and bought it for something ridiculous, £10,000 or so. In London.
At which point, they extended into the garden, creating a 3 bed flat from the ground floor shop, and extending both flats by one 15' x 15' room. Each flat was then given a 999 year leasehold. The ex-shop flat owned equally between the two.
He still rents out his original flat. The other leaseholder sold original flat. But both still let out the ex-shop flat.
Incredible good fortune to land with such an opportunity, OP may too, who knows?
Long story short. It might well be worth the hassle, and expense, of OP fully investigating this.
