The answer
Must radiators be huge?
Radiators do not automatically have to be enormous. They need enough surface area to heat each room at the water temperature used in the heat-pump design. Some existing radiators will be adequate; others may need to be changed.
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Start with the heat each room needs
The designer should calculate heat loss for each room on a cold design day. That calculation uses the room dimensions, walls, windows, ventilation and target temperature. One total for the whole home helps size the heat pump, but it does not tell you whether the north bedroom's radiator can heat that bedroom. Ask for a room-by-room schedule you can read.
Next, identify each existing radiator's size and type. Its advertised output is only meaningful at stated water and room temperatures. A radiator that was sufficient with hot boiler water may give much less heat at the cooler water a heat pump is designed to circulate. The comparison must use the proposed flow and return temperatures, not just a visual judgment that the radiator 'looks big'.
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What could happen to your existing radiators?
Some may stay exactly as they are. Others might be replaced with a double- or triple-panel model that gives more output without using much more wall width. Where wall space is tight, the designer may consider a different emitter, an insulation improvement that reduces the room's load, or a different circuit design. It is not credible to assume either that every radiator must go or that every one will stay without the calculations.
Pipework matters as well. A bigger panel cannot deliver its rated heat if the circuit cannot carry enough water through it. The designer should flag any pipe changes, balancing work and access needed to make the proposed emitter schedule work. Energy Saving Trust also notes that underfloor heating can be an option; it is not a condition for fitting a heat pump.
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How do you read a radiator proposal?
For each room, look for the heat-loss figure, existing emitter, proposed emitter, the water-temperature assumptions and any reason for a change. If the plan depends on insulation you have not yet agreed to install, ask for the numbers for the home as it is now too. A quote that lists 'radiator upgrades' as one lump sum gives you little way to judge the scope.
After installation the circuit still needs commissioning and balancing. At handover, ask which rooms were checked at the design settings and who to contact if one room consistently falls short. Turning up the water temperature for the entire home may mask one underperforming room while increasing electricity use everywhere else.
Where this guidance comes from
Check the original guidance.
These primary sources support the technical or scheme points above. Rules can change; follow the current source before making a commitment.
Apply this to your property
Find out which radiators could stay.
A free heat-pump survey can compare your room loads and existing emitters before anyone assumes the whole house needs replacing.