Beat the Heat: Air Conditioning in Flats

As summers in the UK grow increasingly sweltering, surviving August in a top-floor flat can feel like living in a greenhouse. Unsurprisingly, more and more leaseholders are looking to install permanent air conditioning.

But if you live in a block of flats, you probably already know the golden rule: you cannot hang an ugly, noisy external condenser box on the outside of the building. It breaches planning permission, violates your lease, and ruins the aesthetic of the block.

So, what’s the solution? Enter “invisible” internal air conditioning.

The “Invisible” Solution

Modern climate control technology has adapted to the restrictions of apartment living. Instead of putting the bulky machinery outside, specialised systems hide it inside.

Water-Cooled Split Systems (e.g. Tekno Point, Powrmatic)

This is the gold standard for flats. The external box is replaced by a compact, internal condenser unit that sits inside a cupboard — your airing cupboard, or under the kitchen sink. Rather than using outside air to cool the hot refrigerant, it uses mains cold water. It plumbs directly into your flat’s water supply and drains the warm wastewater into your normal drainage pipes.

You then have standard, quiet wall-mounted units in your living room and bedrooms, connected to the hidden box by internal pipework.

Through-the-Wall Units (e.g. Olimpia Splendid Unico)

These are all-in-one units mounted on an external-facing internal wall. They don’t have a separate hidden box; instead, they require two large holes (around 160mm each) core-drilled through the brickwork to vent directly outside, finished with flush grilles on the exterior wall.

Important note on external walls: through-the-wall units may not be suitable for modern blocks built with complicated external wall systems (EWS), rain-screen cladding, or specialised insulation layers. Drilling through these complex façades can compromise the building’s fire safety barriers or weatherproofing. If your building has cladding, a water-cooled system is likely your only viable option.

Estimated Costs

Internal AC is a niche product and costs significantly more than standard systems:

• Water-cooled systems: typically £3,500 – £6,500+ fully installed. Remember, these will also increase your water bill when running.

• Through-the-wall units: usually £1,500 – £3,000 fully installed.

The Legal Hurdle: The “Cut or Maim” Restriction

While invisible AC solves the problem of hanging boxes on the outside wall, it does not mean you can just buy one and have it installed tomorrow. You have a lease, and that lease likely contains some very strict clauses.

Almost all residential long leases contain a restrictive covenant prohibiting the leaseholder from making structural alterations. This is often phrased as a restriction against “cutting, maiming, or altering the principal walls or structural timbers” of the building.

Installing an internal AC system almost always triggers this restriction:

• Through-the-wall units require core drilling 160mm holes entirely through the external wall — which belongs to the freeholder, not you.

• Water-cooled split systems require drilling through internal structural walls to run refrigerant piping and condensate drains from the hidden condenser to the wall-mounted units. They also require tapping into the building’s plumbing infrastructure.

You Need a Licence to Alter

Because the installation breaches the “cut or maim” clause, you cannot proceed without formal, written consent from your freeholder, usually administered by their managing agent. That consent takes the form of a legal document called a Licence to Alter.

How to Proceed: Contacting Your Freeholder

Before paying any non-refundable deposits to an AC installer, you must write to your freeholder requesting permission to install an internal air conditioning system, and copy in your managing agent. The permission is the freeholder’s to give.

You cannot simply ask for a “yes or no.” You must provide a full proposal, including:

• Detailed plans of where the units will be located.

• Technical specifications of the equipment, especially detailing noise and vibration levels.

• A method statement from your installer showing exactly where pipes will run, which walls will be drilled, and how they will connect to the plumbing.

Be prepared for the costs: under the terms of most leases, the leaseholder requesting the alteration is responsible for covering the freeholder’s professional costs in reviewing the application. This means you will likely have to pay for the freeholder’s appointed surveyor to review your installer’s plans, and the freeholder’s solicitor to draft the Licence to Alter document.

The Bottom Line

Internal air conditioning is a brilliant way to stay cool in a flat without breaking external planning rules. However, it involves significant internal plumbing and drilling. Do it properly: get a detailed quote from a specialist installer, present the plans to your freeholder, pay the associated legal and surveyor fees, and secure your Licence to Alter before a single tool is lifted.