An overheating assessment in London tests whether a dwelling or flat will overheat using methods such as Approved Document O and CIBSE TM59. Air conditioning can be part of the solution, but Part O expects you to reduce heat gains first. A condenser on a balcony is not an automatic pass.
What the assessment is for
New dwellings and conversions in England must limit overheating under Approved Document O. Assessors use simplified methods or dynamic modelling, often TM59 for homes. London's dense flats, single-aspect units and noise-constrained windows make the test hard. This page is the cooling-and-install companion to the indoor overheating and Part O guide.
The hierarchy is reduce gains, purge ventilation, then mechanical cooling if still required. Specifying splits because the model failed, without looking at glass and ventilation, is how you fight Building Control and still own a noisy balcony unit. Existing homes that simply feel hot are not automatically a Part O case. They are a comfort retrofit. Use the home installation guide and the flats guides.
- Part O is for new dwellings and conversions, not every hot existing flat.
- TM59 is an overheating method. TM44 is an AC inspection. They are not the same.
- A Part O report does not grant planning permission for a condenser.
When cooling is legitimate
Noise, pollution, or security that stops windows opening. Single-aspect west flats. Commercial offices that were never dwellings. In those cases a surveyed split, cassette or ducted system is a real answer, not a cheat.
Planning, lease and outdoor siting remain. A Part O report does not grant planning permission for a condenser on a listed facade. TM44 can still apply if cooling output is over 12 kW on the building's systems. New-build clusters of flats can add up.
London practicalities
Balcony slabs, landlord consents, and neighbour noise. Many London installers already know this dance. Bring the assessment, the lease, and photos of the proposed outdoor position to the survey.
2026 cost illustrations for a single London flat split still sit in a four-figure installed band before any landlord extras. Whole-block strategies are a different tender. Do not buy portable units as a Part O strategy. They are not a modelled cooling system and they dump heat and condensate in the wrong places.
Who to call
Overheating assessors and F-gas installers are different professions. If you need the certificate, book the assessor. If you need plant, book the installer. If you need both, sequence the design so the condenser position is in the model, not added after plaster.
We introduce installers. We do not write TM59 reports from a quote form. Use the quote form for the plant. Keep the assessor on their own instruction.
Frequently asked questions
Does Part O ban air conditioning?
No. It asks you not to rely on it as the first measure. If the dwelling still overheats after gains and ventilation are addressed, cooling can be used. Document it.
Is TM59 the same as TM44?
No. TM59 is a domestic overheating assessment method. TM44 is an inspection of existing air-conditioning systems over 12 kW. Different documents, different duties.
Will a split pass my London overheating model?
Only if it is in the model with realistic capacities, controls and outdoor siting. Adding a unit after a fail without rerunning the assessment is not a strategy.
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.