Airspace development means building new homes on top of an existing building. For the freeholder of a London block, it turns an unused roof into value: you can sell the right to build, or take part in the scheme. Whether it works depends on the structure, the planning route, the leases and the people living below.
What airspace development is
Most London blocks were built to a height that suited their time, not the limit of what the site could hold. Adding one or two storeys creates new homes without buying new land, and without the cost and delay of demolition. The new flats are usually built in a lightweight frame so that the existing building can carry them.
We have done this ourselves. At Buckingham Court in Hendon NW4, the whole block was redeveloped over twelve months: two storeys and ten new flats were added, and the entire building was brought up to new-build specification and environmental standard.
What makes a roof suitable
- Structure. The existing frame and foundations must be able to take the extra load, or be strengthened at reasonable cost. A structural engineer's assessment comes first.
- Access. Materials, cranes or hoists, and workers have to reach the roof safely, ideally without passing through residents' homes.
- Lifts, stairs and services. The new floors need fire escape routes, and often an extended lift, water, drainage and power.
- Neighbours and setting. Daylight, overlooking, conservation areas and the height of nearby buildings all affect what the planners will accept.
The planning routes
There are two ways to get consent in England. The first is a full planning application, which allows the most flexibility in design and size. The second is a permitted development right introduced in 2020, which allows up to two additional storeys of flats on certain detached, purpose-built blocks, subject to prior approval from the council. That right has conditions: among them, the block must be at least three storeys high and have been built between 1 July 1948 and 5 March 2018. Many London schemes still go through full planning, because the conditions rule them out or because a better scheme needs more freedom.
Leaseholders and the lease
Before anything else, check what the freeholder actually owns. Some leases include the roof space in a top-floor flat, or give leaseholders rights over the roof that a scheme would interfere with. Selling the airspace can also give qualifying leaseholders a legal right of first refusal under the Landlord and Tenant Act 1987. A solicitor should review the title and the leases at the start.
Residents will usually be living in the building while the work goes on. A good scheme plans the sequence of work, noise, access and communication around them, and leaves the building better than it found it, often with a new roof, refreshed common parts or an upgraded lift.
How freeholders are paid
- Outright sale of the airspace. You are paid an agreed price for the right to build and step away from the scheme.
- Sale subject to planning. You agree terms now and are paid once consent is granted, with the buyer carrying the planning cost and risk. See selling a site subject to planning.
- Joint venture. You contribute the airspace and keep an interest in the scheme, sharing in the profit when the new homes are sold or let.
A typical sequence
A first look at the building and its title; a structural assessment; a planning strategy and design; consent; contracts with the freeholder and, where needed, the leaseholders; the build; and handover of the new homes. On a block of the size we work on, the build itself is often around a year, as it was at Buckingham Court.
If you own the freehold of a London block and want to know what its roof could hold, see how we work with landowners or tell us about the building.