What is the difference between horizontal and vertical leases?

These aren’t actual legal terms, but simply a convenient way of describing the two different ways in which responsibility for a property divided into flats is split amongst the lease-holders. Thus, horizontal leases are those in which the property concerned is divided from left to right, through the middle, while vertical leases, as the name implies, divide the property equally from top to bottom.
Taking the case of a two-story house split into two flats:
• Where leases divide the property horizontally, the leaseholder on the first floor is responsible for maintaining everything above the line – for example, the roof - while the ground floor leaseholder is responsible for the drains.
• Where the division is vertical, both leaseholders have an equal share in all such liabilities.
The overwhelming majority of new leases are of the vertical type. However, there are still some horizontal ones about, since they were quite popular in the 60s and 70s, when leases were often granted for 199 or even 999 years. Generally speaking, you should try to avoid them at all costs, since real difficulties can arise – for example, if you have a landlord or freeholder who cannot be traced, and the other leaseholder doesn’t want to do the work required on, say, the roof. In that case, you can encounter major problems in trying to get the work done at all.
Most vertical leases on the other hand will clearly state that the freeholder is responsible for arranging to have any remedial work done, while the leaseholders are responsible for paying for it. However, it is common practice for the leaseholders to get together and solve any problems between them.
Needless to say, leases don’t state whether they are horizontal or vertical - but you can easily figure out which type it is by studying the list of responsibilities.