Air conditioning in a loft conversion: heat, siting and pipe routes

How common are loft conversions, and do they overheat?

Common, and yes. A 2019 study by Li, Taylor and Symonds, published in Building Services Engineering Research and Technology, found converted lofts in 10.8% of London dwellings against 5.8% across England, and set out to model how they overheat and how to reduce it, using computer simulations of a London semi. A room under a roof takes heat through the roof covering and any roof windows, and warm air from the floors below rises into it. In Ilford the starting point is the housing itself: the town grew from 10,913 people in 1891 to 78,188 in 1911, mostly in long streets of two-storey terraces, where a converted loft is the one room right under the roof. Sources: Li, Taylor and Symonds; Victoria County History, population; Victoria County History, Ilford.

What should you try before fitting air conditioning?

Shading and night ventilation, on the same study's results. External shutters cut overheating degree-hours by 96% and night-purge ventilation by 89%, and once costs were counted the authors found ventilation strategies the most cost-effective. Outside blinds on the roof windows, plus a loft window and a downstairs one left open together after dark, cost far less than a unit. Air conditioning earns its place where windows cannot be left open, because of security or road noise, which the government's overheating research found to be a common barrier, or where those measures are not enough. Sources: Li, Taylor and Symonds; MHCLG.

Does Part O cover a loft conversion?

No. Approved Document O, in force since 15 June 2022, applies to new residential buildings only, so a loft added to an existing house does not have to be shown not to overheat. Homes made by converting other buildings sit outside it too, for now: in April 2026 Elmhurst Energy reported that the government will carry out a full technical review of Part O, including whether to bring material change of use dwellings into it, with no timetable given. For a loft, keeping the room comfortable is left to the owner. Sources: Planning Portal, Part O; Elmhurst Energy, 2025; Elmhurst Energy, 2026.

Which wall takes the indoor unit in a loft room?

On a vertical wall. That means the gable end in an end-of-terrace or a hip-to-gable conversion, the inside face of a rear dormer, or the low wall under the eaves for a floor-standing console. A wall unit needs a flat upright surface and some clearance above it, so the sloping ceiling is rarely an option. Aim it so the air runs along the room rather than down onto the bed or the desk. The condensate drain has a good fall from the top of the house, which makes the drain the easy part.

How do the pipes reach an outdoor unit two floors down?

Usually at the back, well below the loft. A pitched roof is not permitted development at all (see the planning guide), so the choices are the ground, a bracket fixed to the back wall, or a flat roof such as a rear dormer's top or an outrigger, set back a metre or more from each edge. The pipes come down the outside in trunking or through a cupboard or boxed corner on the floor below. On a terrace or a semi the outdoor unit is the property's single permitted one, so if a bedroom below will want cooling too, plan a multi-split from the start.

Source: Planning Portal.

Are there extra rules for a loft in a conservation area?

For the outdoor unit, yes. Inside a conservation area the unit is barred from any road-facing wall or roof, so the front slope and front wall are out, and in Gidea Park Havering's 2025 management plan (as drafted before its adoption) asks for new external devices on rear elevations only. If your dormer needed planning permission, its decision notice may include conditions, since permitted development rights can be removed that way, so read it before the survey. The streets and towns we cover are listed on our Ilford air conditioning homepage. Sources: Planning Portal; Gidea Park management plan.