The answer is generally “yes” you do. An owner of a freehold building owns the air space above the roof.
The same rule can apply to a lease of a building or the top floor in that building. It depends what the lease says whether it includes the air space. If the lease includes it, the landlord cannot add an extra floor to the building or grant a lease of telecoms equipment or solar panels. So the landlord can deprive itself of potential income by failing to exclude the air space from the letting.
H Waites Ltd v Hambledon Court Ltd (2014) concerned a 1960’s block of flats with garages let on 999 year leases. The landlord wanted to build above the garages. The tenants objected and claimed their leases included the air space above the garages.
The leases did not expressly exclude the air space above the garages. It would have been so much better for the landlord had they done so as it would have saved a trip to court. In the absence of clear wording deciding the matter, the court had to look at the whole wording of the leases and the nature of the letting to answer the question. It concluded that the tenants did own the air above the garages. One factor in their favour was the length of the leases – 999 years. This was seen as similar to owning a freehold. As a transfer of a freehold of a garage would almost invariably include the air space, it was reasonable to assume that a 999 year lease would also include it. In addition, it was likely the tenants would need to rebuild the garages at some point; something impossible without a demise of the airspace.
In the 1960’s when the leases of Hambledon Court were granted, there were no solar panels and mobile telephony so one can forgive the parties for not addressing the issue. In 2014 we know that there is money to be made out of leasing out rights to install such equipment. So if granting a lease consider issues such as whether you want to exclude the air space, who is going to repair the roof and whether the tenant needs a right to access the air space ( if excluded from the letting) in order to repair the roof.