A London mansion flat costs more to cool when the ceilings are high, the lease controls the outside, and the outdoor unit needs scaffold or a managed elevation. A 2026 illustration for one straightforward room, once consent and access are real, often sits in the low thousands, and several rooms move into the mid to high thousands and beyond. That is an illustration, not a price list. The budget you can control is which rooms you cool, and whether a lawful outdoor position exists. Start with the lease and the Leasehold Advisory Service before you pick a fascia.
A bay window and a three-metre ceiling
Mansion flats often have tall rooms, big windows and thick walls. The kilowatt that suits a modern box bedroom under-cools a reception with a three-metre ceiling and a bay. Survey the room you mean to use, not a national floor-area table.
The lightwell, the lease and listed fabric
The outside wall, the lightwell and the roof are often outside your demise or controlled by the lease. Written consent comes before the drill. The Leasehold Advisory Service explains leaseholder alterations in plain language. Planning Portal heat pump rules are a separate check, and listed fabric is a third.
Scaffold, the managing agent and a spare way
A single wall split with a short pipe run and a cooperative freeholder is the lower illustration. Scaffold on a street elevation, a managing agent's process, a condensate pump to a distant stack, trunking that must be hidden, and a board with no spare way each add a lump. Two indoor units on one outdoor unit can still be cheaper than two condensers the block will refuse.
- Cool the rooms you sit in, not every corridor.
- Consent and access can exceed the equipment gap between brands.
- One lawful outdoor spot is the constraint to solve first.
A low number that assumes a forbidden balcony
Compare prices on the same consented position. A low number that assumes a balcony the lease forbids is not a budget, it is a restart. There is no standard budget for a mansion flat. Use the 2026 illustration only as a band: low thousands for one difficult-but-possible room, more for several rooms and scaffold. The lease and the access decide it.
One bedroom done properly
One bedroom that overheats, done properly, beats a cheap multi-room sketch that cannot be installed. External shading and using the windows at night remain available and do not need a condenser. When you do buy, an F-gas certified company, a drain that cannot stain the flat below, and a service path to the outdoor unit are the costs worth keeping. High ceilings and bays need their own heat-gain look.
Section 20 is not the licence to alter
Section 20 consultation is a different question: it arises when the landlord does qualifying works and bills leaseholders. Your own flat's install is usually a licence to alter. If the block is proposing plant for everyone, check current thresholds with the Leasehold Advisory Service rather than guessing a fee. Ask the freeholder or managing agent, against the lease, before you treat a sketch as a price. Planning permission is a separate question.
Frequently asked questions
Can I put the outdoor unit in the lightwell to save scaffold?
Only if airflow, service access, noise and the lease all work. A hidden unit that recirculates heat is not a saving. The survey has to accept the well against the manual.
Core buying guides
Start with these high-intent guides, then dig into the rest of the library.
Related guides
Sources and further reading
How this guide was prepared
This guide was written by the Local AC Installers Editorial Team. We compare official guidance, legislation and established consumer information, separate general information from project-specific advice, and show publication dates and sources. We do not accept installer accreditation claims at face value.