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Retrofitting air conditioning into an existing office

Updated 23 October 2026 · SEO Dons Editorial

What makes it a retrofit rather than a replacement?

A retrofit puts cooling where none existed, so nothing is inherited: no pipe routes, no dedicated circuit, no condensate drain, no plant position. Building Regulations treat the two cases differently. Approved Document L says a replacement fixed building service should be at least as efficient as the Section 6 value. Regulation 28 applies to an existing building with a total useful floor area over 1,000 m2 where the work includes the initial provision of any fixed building services. Swapping an ageing system is covered on the air conditioning replacement page.

How big should the system be?

Part L guidance says a comfort cooling specification should rest on a heat gain calculation based on CIBSE's Guide A, and that systems should not be significantly oversized: in most circumstances, not more than 120% of the design cooling load. It also asks for separate control zones where solar exposure, pattern of use or type of use differ. In an older office that can mean treating a glazed south elevation, an interior open-plan floor and a meeting room as three different duties. The sizing guide explains the kW arithmetic.

What does the building have to accommodate?

Indoor units need space an older office may not have. Daikin's cassette manual limits the ceiling opening to 860 to 910 mm and sets a minimum distance to the floor of 2.7 m to avoid accidental touching. Mitsubishi Electric's ducted unit manual asks for a ceiling more than 2.5 m from the floor. Mitsubishi also describes a cassette with a 290 mm high body as suitable for narrow ceiling spaces and for replacing obsolete equipment in older buildings. These are model-specific figures, so the model's own manual governs.

Openings need sealing as well. Approved Document B says every opening for services through a fire-separating element should be sealed with fire-stopping so the element's fire resistance is not impaired. Drainage is covered in the condensate pumps and drains guide, and power in the guides to electrical supply and electrical capacity for new plant.

Where does outdoor plant go, and who has to agree?

Planning comes first. Lewisham Council says planning permission is needed to install, alter, replace or move air conditioning where any part of the equipment is fixed to the outside of a property. Part 14 of Schedule 2 to the GPDO has non-domestic classes running from solar equipment (Class J) to flues for combined heat and power (Class O) and solar canopies (Class OA), and none for air conditioning. The planning permission guide covers what councils assess.

The lease comes next. Where a lease forbids improvements without consent, section 19(2) of the Landlord and Tenant Act 1927 treats the covenant as subject to a proviso that consent is not to be unreasonably withheld. The landlord can still ask for a reasonable sum for damage and, where an improvement does not add to the letting value, an undertaking to reinstate where that is reasonable. Whether a particular clause is an improvements covenant is a legal question for the tenant's adviser. Ownership also matters later: government guidance on air conditioning inspections says the occupier is responsible for the inspection report on equipment it has installed.

Why survey for asbestos before drilling?

HSE says to assume asbestos is present in a building built or refurbished before 2000, and that installation or refurbishment work disturbing the fabric where asbestos could be hidden needs a more intrusive survey before work can start. That survey involves destructive inspection, so the area must be vacated. A management survey, by contrast, covers places such as ceiling voids and risers.

Regulation 5 of the Control of Asbestos Regulations 2012 bars an employer from work liable to expose employees to asbestos unless it has made a suitable and sufficient assessment, or assumes asbestos is present where there is doubt. The asbestos in HVAC plant guide covers older plant.

How is the work run in an occupied office?

HSE's CDM guidance says contractors must plan, manage and monitor their work with the risks to anyone who might be affected in mind, and that a commercial client must give pre-construction information to every designer and contractor bidding for or appointed to the work. That is where access hours, protected areas and asbestos findings belong. A project lasting longer than 30 working days with more than 20 workers at any one time, or exceeding 500 individual worker days, must be notified to HSE.

The person responsible for fire safety must also review the fire risk assessment when there is a significant change in the premises or its technical measures, under article 9 of the Regulatory Reform (Fire Safety) Order 2005. Phasing by floor or zone is one way to keep a building in use, and the installation timeline guide shows how the stages stack.

What does Part L add to a first-time installation?

In England, in an existing building over 1,000 m2, first-time provision triggers consequential improvements. The guidance says the energy efficiency improvements made to the fixed building services should be worth not less than 10% of the value of the principal works, and that all technically, functionally and economically feasible measures to improve the fabric served should be implemented. Regulation 28(3) says nothing is required if it is not feasible. The guidance also asks for energy submetering when fixed building services are provided or extended, and for each cooling unit to meet the minimum SEER in Table 6.6.

The 2026 edition takes effect on 24 March 2027 in England for most building work, and earlier editions continue to apply to buildings subject to earlier standards. Its 120% sizing limit repeats the 2021 edition's. The Part L guide covers the change, and the commissioning and handover guide covers the notice and records. Readers planning a first-time installation can see the commercial air conditioning installation page and send an enquiry through the site's form.

Sources

Common questions

Does a new office system need a TM44 inspection?

If its effective rated output is more than 12 kW, yes, in England and Wales. Government guidance says such systems must be inspected by an energy assessor at intervals of no more than five years, and a system first put into service on or after 1 January 2008 needed its first inspection within five years. Small units count together, so several wall units can add up to more than 12 kW. The TM44 guide explains the process.

Can heat gain be cut instead of adding cooling?

Part L states that limiting summer solar gain through glazing is meant to reduce the need for air-conditioning or the capacity of any system installed. Orientation, shading and glazing are therefore part of the load assessment. In London, the cooling hierarchy guide covers the planning policy on air conditioning.

Who holds the asbestos information in a leased office?

Regulation 4 of the Control of Asbestos Regulations 2012 puts the duty to manage on every person with an obligation of any extent to maintain or repair the premises, by contract or tenancy. In a lease that can mean the landlord, the tenant or both, so the asbestos register should be requested before a survey is ordered.

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