Just replace the boiler in garden flats: 40-90 square metres.
"A boiler needs replacing" was the condition we originally devised to investigate and solve. Having a garden, small houses are always potential candidates for heat pumps, which are generally the ideal solution. Heat pumps often require some form of thermal or system upgrade. The following are options that need almost no intervention.
Option 1
A smart heat battery is the least intrusive alternative to a gas boiler.
Install a smart heat battery, like the Tepeo ZEB, to supply hot water and heating, exactly as you would install a new boiler.
Approximate cost: £6000
Option 2
Same as above, but combined with a smart immersion tank. The addition of the tank gives you more power output, more flexibility of use and ultimately reduces your bills because you are almost guaranteed to charge only at night.
Approximate cost: £8,500
One last thing, even without extensive refurbishment you can still attempt minimal improvements to the building. Sealing front doors and leaky windows brings amazing returns in investment and comfort. Replacing or increasing the loft insulation does not require any demolition. In some cases, very smart robots can also insulate under the ground floor floorboards through the undercroft. Depending on the condition of the heating system and your budget, you have two options.