A roof lantern heats the top of the room first. The hot layer sits under the glass while the floor can still feel ordinary, until the whole volume catches up. Floor area understates that load. Shade the lantern if the product allows, survey the glass, and plan an air path that stirs the space under the lantern rather than only washing one wall. A single wall unit at the edge can leave the centre warm.
A hot layer under the lantern, and air that has to reach it
Vertical windows add sun on one face. A lantern adds sun at the highest point, which is why galleries and kitchen diners with lanterns overheat even when the walls are modest. Note the lantern size, the orientation, and whether it is clear or solar-control glass. Internal roof blinds help. External shading or solar glass helps more. Include the lantern area in the heat gain. Expect a warm layer at high level. Ceiling height under the lantern is part of the volume. A survey that only uses the floor footprint will undersize the unit. The sun heats the air at high level first, which is why a thermometer at the sofa can still read fine. Cool and mix that volume, or the heat soaks down later.
A wall unit opposite the lantern, set to throw across the high space, is the usual domestic answer. One sidewall indoor may not mix the centre of the room. In a roughly square room with a ceiling void around the lantern, a cassette can spread air four ways, if the void depth and the drain exist. Many house lanterns sit in a plasterboard raft with no useful void, so a cassette is then the wrong hope. Two smaller indoors can bracket a long lantern room more evenly than one large unit. They can share a multi-split outdoor unit when the combination chart allows. A bigger single wall unit fixes the room only if it can actually circulate the space. Very long rooms often prefer two indoors so the centre under the glass is not a dead zone.
Pipes that miss the upstand, and a condenser off the felt
The lantern upstand, the steels and the roof membrane are not places to invent a pipe route. Keep refrigerant lines in agreed voids or surface trunking, insulated, and clear of the glazing drainage. Condensate and pipes must miss the lantern structure. Penetrations through a flat roof need a proper weathering detail. Condensate pumps are common because the indoor unit is far from an external wall. The pump discharge should run to a waste you can see fail, not into the lantern gutter.
The outdoor unit still belongs outside, with airflow and a service gap. Flat roofs next to lanterns are tempting and need a structure and waterproofing check, not a condenser stood on the deck. It can sit beside the lantern only on a frame the structure can carry, weathered properly, with safe access for service. It must not be stood loose on the roof covering. F-gas rules apply. In England, follow Planning Portal guidance for the heat pump. A lantern does not change neighbour noise. If the open-plan room is large enough that you are leaving domestic splits behind, check whether the finished system could exceed 12 kW and fall under GOV.UK inspection rules.
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.