For a typical UK house split, surveys often allow a core of about 65 mm through masonry so an insulated pipe pair and a cable can pass with room to seal. That is a planning figure for the installer, not a diameter to buy a drill for. Pipe size, insulation thickness, a bundled drain, stone, timber frame and a listed wall all change it. The survey confirms the hole. The installation team forms it, sleeves it and makes it good.
One sleeved opening for the lineset
The outdoor and indoor units need a short, sleeved route for the lineset and the interconnecting cable. On many domestic jobs that route is one neat opening in the external wall, high enough to suit the indoor unit and close to the outdoor position. A separate small opening sometimes takes a gravity drain. The aim is a weather-tight sleeve, a fall that does not track rain indoors, and a seal that suits the wall type. Fire stopping matters where the wall is a compartment. None of that is a weekend drilling task for the householder. About 65 mm is a common domestic allowance, confirmed on survey. The installer drills, sleeves and seals.
When 65 mm moves, and when the place is wrong
A small wall split with standard insulation often passes a 65 mm core with a little spare. Step up the gas line, add thicker insulation for a long heating run, or try to drag a condensate pipe through the same hole, and the surveyor may call a larger core or a second hole. Larger commercial pairs and thick insulation can need a larger opening. Forcing pipes through a tight opening tears the insulation and leaves a cold copper line sweating into the plaster. Wall build changes the method more than the brochure does. A cavity wall, a solid Victorian brick wall, a stone cottage, a timber-frame panel and a concrete lintel are four different jobs. The survey looks for cables, pipes and the cavity closer before anyone commits to a spot. A perfect 65 mm hole in the wrong place is still wrong. Party walls, neighbour windows, flue outlets, gas pipes and the only path across a listed frontage all sit above the millimetre conversation. Listed fabric, party walls and concealed services can rule a spot out. The Planning Portal page on air source heat pumps is the heating-plant starting point for outdoor equipment. A core through a protected wall can need consent even when the outdoor unit itself looks modest. Inside, the hole should land where trunking or a cupboard can hide the pipes, not in the middle of a feature wall. Agree that route on the drawing. Moving it after decoration is the expensive version of the same hole. Often the hole is part of an ordinary install on a house the owner occupies. Flats, listed buildings, conservation areas and party walls can be different. Check outdoor-unit permission and any building consent before the date is booked.
Making good, stone and the joints after the sleeve
The quote should say who drills, what making good is included, and whether internal trunking or a builder's chase is assumed. Diamond drilling thick stone or concrete is often an extra. As a 2026 illustration, that extra is commonly talked about in the low hundreds of pounds including VAT when access is simple, and more when scaffolding or a party-wall agreement appears. Your survey is the price. Ask for the proposed diameter as an allowance, plus a note that it will be confirmed against the pipe insulation and the wall. The refrigerant connections themselves stay with the F-gas certified installer after the sleeve is in. GOV.UK sets out certification for companies that work on that circuit. Do not drill the hole yourself before the installer arrives. The survey confirms diameter, position, fall and services. A hole in the wrong spot, or through a cable, costs more than waiting.
Frequently asked questions
Should I drill the 65 mm hole myself before the installer arrives?
No. The survey confirms diameter, position, fall and services. A hole in the wrong spot, or through a cable, costs more than waiting. Drilling is part of the installation.
Is 65 mm always enough?
It is a common domestic allowance for an insulated small-split pair and a cable. Larger pipes, thicker insulation or a drain sharing the hole can need more. The survey writes the figure for your wall.
Do I need consent to core an external wall?
Often the hole is part of an ordinary install on a house the owner occupies. Flats, listed buildings, conservation areas and party walls can be different. Check outdoor-unit permission and any building consent before the date is booked.
Who forms the hole and who connects the pipes?
The installation team drills, sleeves and seals. The refrigerant connections stay with the F-gas certified installer after the sleeve is in. Fire stopping matters where the wall is a compartment.
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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.