Posted by: @transparent

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Are the civil servants thinking of a domestic turbine within  a suburban garden?...
... in which case the definition of "curtilage" needs specifying

Yes. The pd is for heat pumps on 'domestic premises'.  The term curtilage is frequently used on planning law and, to the best of my knowledge, just means the property boundary.

So a farmhouse isn't "domestic premises"?
Is that being defined because there is or isnt a boundary wall between the house and the agricultural buildings?
Or because it may or may not have its own electricity supply?

I do have a wind turbine within the curtilage of my own home. But the farm buildings are ¼mile away in the valley.

Posted by: @transparent

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So a farmhouse isn't "domestic premises"?

Without doing some research I can't immediately answer that question in planning law, but I would think it almost certain that the matter has been settled by one or more cases, albeit that case law continues to evolve.

The meaning of curtilage (also in practice a matter for case law) is discussed here

@transparent

Further to the above the actual text, describing what is permitted, reads

Permitted Development

G.  The installation, alteration or replacement of a microgeneration air source heat pump—

(a)on a dwellinghouse or a block of flats; or

(b)within the curtilage of a dwellinghouse or a block of flats, including on a building within that curtilage.

Thus, although the title uses the word domestic, the actual text doesn't refer to 'domestic premises', it refers to 'dwelling houses' and 'flats'. 

The interpretation section of the same order has something to say about the meaning of these terms and the Use Classes Order 1987 will also be relevant, possibly definitive.  

A farmhouse, if principally designed for living in, would be a dwelling house in my reading.  If it were designed for mixed use it may be questionable.  The definition of curtilage therefore becomes relevant and I posted a link where there is discussion and links to some relevant case law.  In essence it's context dependent.  Hopefully this may give some indication whether a turbine 0.25 miles away on land in the same ownership would negate the installation of a heat pump or not under pd.

Posted by: @jamespa

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I suppose it could be a noise argument?

Posted by: @jamespa

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Don't forget that permitted development means no opportunity for the lpa to review/intervene so tends to be conservative.

I'd be surprised if it wasn't a noise consideration. Neither wind turbines nor heat pumps are intrinsically noisy but both have come under attack in the past for the potential to be so in certain circumstances. In that light, it would be a pretty bold stance to allow two potentially noise-creating items to be installed close by one another without requiring some sort of sanity check that they won't compound each other's problem.

This is, of course, only permitted development. The fact remains that heat pumps and wind turbines can still be installed close to each other with planning permission.

All my own supposition, of course...

Sound level calculation for air source heat pump permitted development noise compliance using MCS 020 a) against the fixed 37 dB(A) limit.
See this useful online calculator:
https://tradecalculator.co.uk/renewables/heat-pump-noise-calculator/
For example, my two Daikin A2A outdoor units (sited side-by-side) have a maximum sound power of 60dB each, they are on the ground with a wall behind them (Q4).
There are two assessment positions that need to be considered: (i) 1m perpendicular in front of the neighbour's back door, which is 9m away, and (ii) 1m perpendicular in front of the neighbour's nearest first floor window, which is 14m away. Position (i) is completely obscured by a solid fence (<18mm thick) and position (ii) is in full view. Due to the sound damping effect of the fence (-5dB) both positions need to be checked.
The calculated noise levels are (i) 34dB and (ii) 35.1dB, so both are within the 37dB limit.
Note, if the fence wasn't there, then the noise level would be 39dB at position (i) and would not be compliant. Having a single heat pump and no fence, the maximum noise level would be 35.9dB, which would again be compliant.
The calculator is useful for checking compliance in simple situations i.e. one or two heat pumps. If you scroll down on the webpage, it explains the assumptions of the model and the formula that MCS 020 a) uses.
In practice as a courtesy to my neighbours, and for maximum efficiency, I only ever run the heat pumps in eco mode which drops the maximum sound level substantially by reducing the maximum compressor and fan speeds. That does not however make any difference to the compliance calculation required.

The idealist in me says pioneers in this space should be rewarded!