Rooftop Airspace Value Calculator
Work out the residual value of rooftop airspace from the end value of new units less build cost, fees, consents, sale costs and profit.
💼 Are you a real estate agent or broker? Send this calculator to your buyers on your own branded listing page — every enquiry comes straight to your WhatsApp. Create free agent page →Airspace above an existing building can be developed into new units without buying land, which sounds like free value and frequently is not. Building on an occupied roof is expensive — access, craneage, structural strengthening, working around residents — and the consents needed from leaseholders and freeholders take time and money.
The residual approach is the same as for any development site: work back from what the finished units are worth to what the airspace can be worth.
The residual approach
Start from the combined value of the finished units. Deduct the build cost with fees and contingency, the sale and marketing costs, the developer profit, and the cost of obtaining consents from freeholders, leaseholders and the planning authority. What remains is what the airspace can be worth.
Worked example
Two units worth 320,000 each is 640,000 of end value. At 700 square feet each and a 240 build rate the base cost is 336,000, and with 12% fees and 10% contingency that becomes 413,952. Sale costs at 3% are 19,200, profit at 20% is 128,000, and consents 25,000 — leaving an airspace value of about 53,848, or 8.4% of the end value.
Build rates on roofs are higher
Craneage, restricted access, working above occupied homes, structural strengthening and out-of-hours restrictions all push rooftop construction well above ground-level rates. Using a normal build rate is the fastest way to make an unviable scheme look viable.
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Create your free property pageFrequently asked questions
Who owns the airspace above a building?
Usually the freeholder, though leases sometimes grant rights over the roof and local law varies considerably. Establishing ownership and what consents are needed is the first step, well before any appraisal.
Why is rooftop building so expensive?
Access and craneage, structural strengthening of a building not designed to carry more, working around occupants with restricted hours, and lifting every material to height. Rates well above ground-level construction are normal.
Do existing residents have to agree?
Frequently yes, in some form — lease consents, a right of first refusal, or statutory rights over common parts. Resident opposition is a common reason rooftop schemes fail even where planning is granted.