Air Conditioning in London Renovations: Split Systems, Cassettes, and Planning Constraints

The number of London residential renovations specifying air conditioning has increased significantly over the last decade, driven by hotter summers and the thermal mass of Victorian brick construction that retains heat into the evening. In a well-specified renovation, air conditioning is invisible — the indoor unit is integrated into the ceiling or concealed behind a decorative grille, the refrigerant pipework is fully concealed within the fabric, and the outdoor unit is positioned where it is not seen from the street. Achieving this requires planning at first fix stage, not as an afterthought after the walls are closed.

This guide covers the principal AC system types used in London residential renovation, planning constraints, and what to specify at design stage.

System types

Wall-mounted split systems

The most common and lowest-cost configuration. The outdoor condensing unit (typically wall-mounted or ground-mounted at the rear of the property) is connected by refrigerant pipework to an indoor wall-mounted head unit. Wall-mounted heads are functional and increasingly slim-profiled, but they are visible architectural elements — the quality of their integration into the room is a design consideration.

For a principal bedroom or formal reception room, a wall-mounted head is generally considered visually intrusive at the level of finish the room receives. In secondary rooms (spare bedrooms, home offices, utility areas), wall-mounted units are an entirely acceptable specification and significantly cheaper than concealed alternatives.

Fan coil units: a more premium wall or ceiling-integrated option. Fan coil units are connected to a chilled water circuit (from a central chiller plant) rather than direct refrigerant. They are quieter, longer-lasting, and more easily concealed than direct refrigerant units, but require a separate chiller plant and hydraulic distribution system — appropriate for whole-house systems in larger properties, not typically specified in a single-room application.

Ceiling cassette units

A ceiling-recessed unit that distributes conditioned air through a flush ceiling grille. The cassette body is above the ceiling; only the grille is visible. This is the preferred specification for open-plan spaces (kitchen-diners, ground floor reception rooms) where a wall-mounted unit would be visible across the full width of the room.

Technical requirements: the cassette requires a ceiling void of minimum 280–320mm to accommodate the unit body and refrigerant connections. In a Victorian terrace with 2,600mm ceiling heights, creating this void through a new suspended ceiling at cassette level reduces the room height to approximately 2,280mm — acceptable but worth modelling against the room proportions before committing. In a contemporary rear extension with a flat roof, the void within the roof build-up can often accommodate the unit without reducing internal height.

Drainage: cassette units produce condensate (water removed from the humid air) that must drain away. In most installations this requires a condensate pump (the cassette is typically below the level of a gravity drain). The pump must be connected to a drain — typically a sink waste or soil pipe. Specify the condensate drain route at first fix stage.

Concealed ducted systems

The highest-specification option for primary rooms. A concealed air handling unit (AHU) is located in a ceiling void, loft space, or plant cupboard. Insulated ductwork distributes conditioned air to supply grilles positioned flush in the ceiling or high on the wall. Return air is drawn back through a separate return grille. The system is entirely invisible in the room — no unit, no pipework, only slim linear or circular grilles.

Advantages: fully invisible; the air distribution is even (multiple supply grilles rather than one unit); the plant can serve multiple rooms from a single outdoor unit.

Technical requirements: significant ceiling void is required for ductwork (minimum 300–350mm for main supply duct runs, reducing to 200mm at branches). This system must be designed and coordinated at architectural stage — it cannot be retrofitted into a completed interior without extensive disruption. For a new extension or loft conversion, it is the natural specification.

Cost: significantly higher than split or cassette systems. A concealed ducted system serving 3–4 rooms from a single external unit: £15,000–£35,000 installed depending on complexity.

Planning constraints

External unit visibility: in conservation areas, an air conditioning outdoor unit on a principal elevation visible from a public road requires planning permission and is frequently refused on visual grounds. The outdoor unit must be positioned on the rear elevation or in a roof void/plant space where it is not visible from the street. This constraint often determines the refrigerant pipe route — a long run from a rear-positioned outdoor unit to a front bedroom has implications for pipe concealment and system efficiency.

Permitted development: in England, air conditioning outdoor units on domestic dwellings are generally permitted development (no planning application required) subject to conditions: not on a wall or roof visible from a highway, not on a listed building, must meet noise requirements. In a conservation area, any unit visible from a road requires a planning application.

Listed buildings: all external changes require Listed Building Consent. Routing refrigerant pipework through external walls also requires LBC if it involves drilling through listed fabric. Early pre-application consultation with the local authority conservation officer is essential.

Noise: outdoor condensing units produce noise at approximately 40–55dB(A) at 1m depending on the model. Position the outdoor unit away from neighbouring bedroom windows and in compliance with the 45dB(A) noise limit at the property boundary specified in MCS MIS 3006 for heat pumps (a useful proxy for AC units in residential settings). Low-noise models are available for sensitive locations.

Refrigerant pipework concealment

The refrigerant pipework connecting indoor and outdoor units must be concealed within the building fabric in any quality renovation. Surface-run pipework in trunking on an external or internal wall is not acceptable in a premium interior.

Pipework runs should be agreed and marked on drawings before first fix — the pipe chases must be cut (in masonry) or the routing planned through stud walls and ceiling voids before plastering. In a multi-storey property, the routing from a ground floor external wall to upper floor rooms requires either a vertical chase through the building (coordinated with structural elements) or a route through ceiling voids at each floor level.

The two pipes (flow and return refrigerant) plus the condensate drain and electrical cable are typically run together as a bundled service. Pipe diameters range from 6.35mm to 15.88mm depending on the system capacity — confirm with the AC engineer before first fix.

Specification: efficiency and refrigerant type

Modern AC systems use R32 or R410A refrigerant. R32 has a lower global warming potential (GWP 675 vs 2,088 for R410A) and is now the dominant refrigerant in new equipment. Specify R32 equipment as standard — R410A equipment will be progressively phased out under F-gas regulations.

SCOP and SEER ratings: the seasonal coefficient of performance (SCOP for heating, SEER for cooling) indicates energy efficiency. For a quality specification, target SEER ≥6.0 (ErP A+++ in cooling) and SCOP ≥4.0 (ErP A+ in heating). Premium units from Daikin, Mitsubishi Electric, and LG achieve these ratings as standard.

Inverter technology: all modern split systems use inverter-driven compressors that modulate output to match the load. This is the correct specification — fixed-speed (non-inverter) units are less efficient and less comfortable. Confirm inverter specification when purchasing.

ASAAN's experience

ASAAN has carried out AC system installation and servicing on prime London residential properties, including high-value estate projects requiring careful integration with existing luxury interiors and sensitive management of planning constraints. Works have included concealed ducted system installation in conservation area properties and routine AC servicing and maintenance on multi-room residential systems.