Can You Install Air Conditioning in a Flat? What to Check First

Who needs to say yes before you start

Most flats in South West London are leasehold, and the lease almost always requires the freeholder's or management company's written consent before you fix anything to the outside of the building. This is the step people skip and then regret. A managing agent will typically want a short method statement, a picture of the proposed outdoor unit position and confirmation the installer is F-Gas registered, which is a legal requirement for anyone handling refrigerant.

Consent can take anywhere from a couple of weeks to a couple of months depending on how responsive the agent is, so start that conversation before you book an installation date. If you own a share of freehold, you still need the other shareholders to agree. Council tenants and housing association residents should assume permission will be refused unless there is a medical reason.

Planning permission is a separate question. Fitting an outdoor unit to a flat is not covered by permitted development the way it can be for a house, so in theory a planning application is needed. In practice Hammersmith and Fulham treats a small unit at the rear or on a private balcony as minor, but anything on a front elevation or in a conservation area needs care. Listed buildings need listed building consent regardless.

Where the outdoor unit can go

Every standard split system needs an outdoor condenser unit. In a flat the usual options are a balcony, a flat roof you have rights over, a wall bracket on the rear elevation, or occasionally a light well. The unit needs clear airflow, so boxing it into a tight balcony corner or behind planters reduces efficiency and shortens its life.

The distance between the indoor and outdoor units matters too. Most domestic systems allow a pipe run of around 15 to 20 metres and a height difference of 5 to 10 metres, which is fine for most flats but can rule out a ground floor unit serving a fourth floor bedroom. The pipework itself is about 30 to 40mm across once lagged, and it needs a route through the external wall and a fall for the condensate drain.

• Balcony: easiest, provided the lease lets you fix to the balcony wall or floor
• Rear wall bracket: common in Victorian conversions, needs freeholder consent and safe access
• Flat roof: good airflow, but check you have demised rights to that roof space
• Shared courtyard or garden: rarely permitted because of noise to neighbours

If an outdoor unit is not an option

Some flats simply cannot take an outdoor unit, either because the lease forbids it or there is nowhere sensible to put one. There are two workable alternatives. A through wall or window unit sits in a single box with vents ducted through the wall, and needs two core drilled holes of roughly 160mm each. It avoids an external unit but is noisier indoors and less efficient than a split.

The other option is a portable unit, which needs no installation at all but is loud, uses a lot of electricity and struggles to cool a room above about 20 square metres. Portable units are a fair stopgap for one hot bedroom, not a substitute for a fixed system. Be wary of any installer who claims a portable is equivalent to a split system.

What it costs and what affects the price

For a single wall mounted split system in a flat, a realistic installed range in this part of London is roughly £1,800 to £3,000 depending on the unit size, brand and how awkward the pipe run is. A multi split serving two or three rooms from one outdoor unit usually lands somewhere between £3,500 and £6,500. Scaffolding or a tower for a high rear wall, long pipe runs, or an electrical supply upgrade all push the figure up.

Running costs are lower than most people expect. A modern 2.5kW unit cooling a bedroom draws around 0.7kW when working, so an evening of use costs well under a pound at current tariffs. The same unit also heats efficiently in winter, which is often what tips people into fitting one.

• Unit capacity: 2.5kW suits a typical bedroom, 3.5kW for a living room of 25 to 30 square metres
• Access: ground and first floor rear walls are simplest, anything higher may need access equipment
• Electrics: most units run from a dedicated 13A or 16A spur, older flats may need a consumer unit upgrade
• Finish: trunking on the outside wall versus concealed pipework inside a boxed run

How a good installation should go

Expect a site survey first, ideally in person, so the installer can check wall construction, the pipe route and where the condensate will drain to. A good survey will also flag anything the freeholder is likely to query. Once consent is in, a single split typically takes one day to fit, and a multi split two days. The system is pressure tested with nitrogen and vacuumed before refrigerant is released, and you should receive an F-Gas commissioning record and a manufacturer warranty registration.

Annual servicing keeps the warranty valid and the filters clean, which in a city flat matters more than people think. Blocked filters are the single most common cause of poor cooling and the smell people sometimes complain about.