Heating & cooling

Consent — largely solved

• Share of freehold, and you are a director of the company. That removes the slowest and least predictable item on the list. You still want the company’s agreement minuted rather than assumed, but you are asking colleagues, not a landlord with no incentive to reply.
• Exclusive use of the garden and the side passage. Both ground-level positions are yours to use. No demise argument to have.
• The roof needs the other owners, which makes it the slow option rather than the impossible one.

Permitted development — available

• No heat pumps in the building, so yours is the first — which is the one that qualifies. Confirmed.
• Four flats and nothing commercial, so the block is wholly residential and the right applies.

• Conservation area is the last open question, and it takes about a minute. Search your council’s planning constraints map by postcode, or the planning portal for your borough — most publish an interactive layer. It would not stop the installation; it would stop the unit going on any wall or roof fronting a highway, which the garden position avoids anyway.

Going first uses up the building’s allowance

• Only the first installation in the building is permitted development. Once yours is in, the other three flats each need a full planning application to fit one.
• That is worth knowing as a director rather than discovering later. It is not a reason to hold off — the allowance goes to whoever moves first either way — but it is the kind of thing that causes friction at an AGM in three years if nobody was told.
• There is a constructive version: raise it now, and if another flat is thinking about it, a joint application or an agreed position for future units is far easier to get through than four separate ones later.

Permitted development, as it applies to a flat

Noise is measured at your neighbour’s window

• The limit is 37 dB L_Aeq,5min, measured 1 m outside the centre of a neighbouring habitable room door or window — not at your boundary, and not at your own wall.
• The 1 m boundary rule was removed in 2025, which gives you more placement freedom than older guidance suggests. From 28 May 2026 MCS 020 is the only certification route the Planning Portal recognises, so the calculation is now the test.
• The side passage is the wrong instinct. A narrow passage between two buildings is a reflective corridor — sound bounces rather than disperses, and a neighbour’s window is likely to be close and directly in line. The garden will almost always calculate better than the passage, even if the passage looks tidier.
• Ask any installer for the MCS 020 calculation with the assessment positions marked, not a verbal assurance. It starts from the manufacturer’s sound power data and it either passes or it does not.

The grant

• The Boiler Upgrade Scheme is £7,500 for an air source heat pump. The £9,000 rate that runs to March 2027 is only for off-gas-grid properties, so it does not apply here — you are on gas and taking it out.
• Conditions: own the property, be replacing a fossil fuel system (your boiler qualifies), and use an MCS-certified installer in an approved consumer code. The installer claims it and deducts it from the quote — you never handle the money.
• There is no mandatory insulation or EPC condition attached to the grant as it stands, though installers will often recommend fabric work first anyway.
• Leasehold is not addressed in the headline guidance — worth confirming with the installer early, because it interacts with the freeholder consent question rather than replacing it.

Cooling — and why it is a heating decision

• You have to decide this now, not later. Only the first outdoor unit in the building is permitted development. Fit an air-to-water heat pump now and bolt a separate air conditioning system on in two years, and that second outdoor unit needs a full planning application — on a building where you have already used the allowance.
• That is the real efficiency you were hoping for, and it is bigger than any equipment saving: one outdoor unit, one planning allowance, one set of pipework, one set of holes in the wall.

What actually cools the flat

• Underfloor cooling does not really work, and it is worth saying plainly. The floor cannot go below the dew point or it condenses, which caps output at roughly 15–25 W/m² — less again through an engineered board. Against a room with large glazing at 60–100 W/m² of load, that is about 0.8 kW of cooling against a 2.7 kW load across both rooms. It takes the edge off a warm evening. It is not air conditioning.
• Worse, condensation under a timber floor is exactly the failure you do not want. Underfloor cooling and engineered wood is a combination most manufacturers will not warrant.
• Fan coils are what actually cool — they dehumidify as well as cool, which is most of what makes a room feel comfortable. They are visible boxes and they need a condensate drain, which is a thing to plan while the floor is open, not after.

The spec changes if cooling is in scope

• Pipework insulation. Heating-only insulation is about stopping heat loss. Cooling pipework must be vapour-sealed or it sweats and drips inside walls and floors. Different, more expensive spec — and if you install heating-grade now and add cooling later, much of it has to come out.
• Condensate drainage to every fan coil position.
• The heat pump itself must be a reversible model, and not all are. This narrows the shortlist, and it interacts with the sound power question — you want a quiet reversible unit, which is a smaller list again.
• Check the grant. Reversible units are generally eligible for the Boiler Upgrade Scheme, since it is assessed on the heating and hot water function — but confirm with the installer before assuming.

The cheaper half of the answer

• Two full-height windows on external walls will drive the summer temperature more than anything the plant does. External shading or solar-control glazing removes heat before it arrives, costs nothing to run, and reduces the cooling capacity you need to buy.
• Ground floor with a garden also means night purge ventilation is available to you in a way it is not for an upstairs flat. Openable windows at both ends of the space, used overnight, do real work in a London summer.
• Do these first and the cooling system gets smaller, quieter and cheaper. Do them second and you have bought capacity you did not need.

The flat is bigger than the drawings

• The lease plan shows a full flat — reception, kitchen, several bedrooms, bathroom and corridor, plus a large tapering garden. Everything drawn so far covers roughly 38 m² of it. The heat pump sizes for the whole flat, not the renovated part, so the heat loss calculation has to cover rooms nobody is otherwise touching.
• Two good pieces of news from it: the garden is generous and tapers away, which gives you all the distance the noise calculation could want; and the demise is clear.
• The communal strip is a fire escape route, and the unit would sit further down in your exclusive area — so no conflict. Worth noting for the installer though: nothing can be stored or sited so it narrows an escape route, including during the works, and that includes pipe runs at low level.

The hidden cost: the radiators you are not replacing

• Underfloor in the living and kitchen, radiators everywhere else is a mixed system, and the radiators are what set the flow temperature for the entire flat. A heat pump running at 45°C to satisfy an undersized radiator is running at 45°C for the underfloor too, every day, for twenty years.
• Existing radiators sized for a 75°C boiler deliver roughly a third of their rated output at heat pump temperatures. Most will need replacing with something two to three times larger — in bedrooms and a corridor that are not otherwise part of this renovation. That is likely the largest unbudgeted item in the heating plan.
• The alternative is running the system hot enough for the old radiators, which quietly costs you 20–30% on every unit of heat for the life of the system. It is the classic way a heat pump ends up with a bad reputation.
• Worth asking for the heat loss calculation to state, room by room, what flow temperature each existing radiator would need. That single column tells you which ones actually have to change.

Can underfloor replace the 3 m radiator?

• The built-in joinery eats a quarter of the floor. Underfloor cannot go beneath fixed units, so 31.8 m² of room is 23.7 m² of heated floor. That is the number nobody quotes and it is why floor-only schemes come up short.
• On these figures underfloor alone probably does not replace that radiator. Best case it matches a single panel; against a double it is roughly half. The radiator is very likely oversized for the real heat loss — most are — but the gap is too big to assume away.
• Two full-height windows, garden doors and a solid-wall Victorian half do not help. Fabric first genuinely changes this answer: bring the heat loss down and the floor becomes sufficient. Leave it and you are choosing between a cold room and a hot floor.

The fan coil is the answer to the radiator

• You are already considering a fan coil in the living area for cooling. The same unit heats. That is the elegant resolution: underfloor carries the base load, the fan coil covers the coldest days, and the 3 m radiator still goes.
• It also fixes underfloor’s real weakness, which is response time. A low-profile system under timber is slow; a fan coil brings the room up in minutes when you want it.
• And it means the cooling spend is not a luxury line item — it is doing structural work in the heating design. That materially changes how the cooling budget reads.
• Practical consequence: the fan coil position needs deciding with the joinery, not after. It wants to be high, out of the party-wall build-in, and it needs a condensate drain.

The dining room floor stays down

• So no underfloor there, and its radiator stays — which means it is one of the emitters that will need upsizing for heat pump flow temperatures. In a room where the murphy bed takes 165 of the 178 wall, finding space for a radiator two to three times bigger is not trivial. Worth checking early, because it competes with the bed.
• You are replastering and re-skirting in there anyway. Run conduit at skirting level while it is open — not for underfloor, but for a future fan coil or a relocated emitter. Cheap now, disruptive later.
• With the dining floor staying down, the living and kitchen floor is the only open floor in the flat. Everything that ever needs to cross the building has to be laid there, or through the hall.

The priority order runs backwards to the works

• Only one floor opens, and it is priority 1. Every other zone — including the dining room you are renovating — keeps its floor. So four of five zones can only ever be served through the hall.
• The hall is the pinch point. The plant and the cylinder are at the back; the front rooms are at the front; everything has to cross the hall, and the hall is not in scope. Once both renovated rooms are finished, the hall floor is the only route left and it means lifting a finished floor to serve a finished room.
• So channel the hall now. One strip of hall floor lifted while everything either side is already open, with pipe, conduit and a condensate route laid in and capped, is cheap. Doing the same job in two years costs the hall floor, the front rooms’ skirtings and a fortnight. This is the highest-value thing on this page and it is not a heating decision, it is a five-metre channel.
• Then fit fan coils to priority 1 only if budget is tight. It is the most glazed and most occupied space, cool air migrates, and the stubs are waiting for the rest.

What already constrains the design

• Low-profile underfloor, not screed — forced by the 2200 bulkhead. Less output per m² and far less thermal mass than installers usually assume.
• Engineered board under 18 mm, to stay under 0.15 m²K/W.
• Cylinder boxed in at roughly 520 × 860 — about 180 litres in a shape most heat pump cylinders are not made in. Coil surface area needs checking for low flow temperatures.
• Two full-height windows on external walls in the coldest corner, which will dominate the kitchen’s heat loss.

This week, in order

• Measure the passage width, and pace to the nearest window across the fence.
• Get a whole-flat room schedule together — every room, its floor area, and the size and type of the radiator in it. That is the input for the heat loss calculation and it is the thing that decides the flow temperature.
• Then, and only then, get two MCS installers to quote — each with the MCS 020 calculation and a room-by-room heat loss, not a rule-of-thumb size.