@jamespa And those like me who needed something of a slightly different ‘flavour’ due to space limitations and access. Regards, Toodles.

@jamespa I agree it's sensible - now I know what their strategy is and it's not just a huge error. As I posted earlier they will be offering a much larger HP next year which likely would be acceptable. If I care to wait that long. British Gas are not doing installs in my area so my options are now very limited.

@davesoa To me, there is something just a little ironic about a purveyor of fossil derived energy promoting and installing renewable energy driven heating systems! 😉 (He-He) [If you can’t beat ‘em..] Regards, Toodles.

@davesoa is that a single run of wall in one room is mixed construction? They can definitely deal with different walls being of different construction as I have both solid and cavity walls - unless the octopus surveyor marked everything as cavity and my heat loss is now underestimated!

@gunboatdiplomat A little more complicated. I have one small room that is entirely solid wall, one large room with one wall run that is about 15% solid wall with the rest of that particular run cavity wall (the other walls are insulated), one room that is a mix, 75% solid (balance insulated to 2004 regs), one room that is 50% solid.

@davesoa Ah yeah that is a bit more complex. Though in my mind it's easy to cater for - calculate the average u-value for the wall and use that. Very frustrating if Octopus won't allow for that - it's not hard and it's the way I had to make things work in the MCS calculator spreadsheet as it has one field for external walls.
 

@gunboatdiplomat I think some users are wedded to their computers and software, and not allowed to think for themselves - more’s the pity. I spoke to someone a few days back in an attempt to ‘register’ a new freezer. When I eventually got through all the ‘press one for this, press two for this’ nonsense and spoke to the person to carry out the registration, it soon became obvious that it was a data gathering exercise and when asked for my first name, they would not accept just an initial telling me the computer won’t allow them to put in an initial only, to which I told them tough, that’s all I am offering, they persisted, I rang off. Regards, Toodles.

Does anyone have any experience of planning requirements for a heat pump which is dimensionally over the 0.6m3 permitted development limit? British Gas have quoted me for a 12kW Valliant (0.7m3), but won't install it until I have planning permission (having sat on my deposit for six months without telling me this). 
I have started to fill in an application for a Lawful Development Certificate, but it seems to me the Bristol City Council are likely to say 'no', it's over 0.6m3 so apply for planning permission. 
Out of interest (and frustration), and to inform the original debate here are my quotes so far:

Octopus quoted £12k for a 14kW Daikin. I paid the deposit after the survey, but then Octopus pulled out on 'heat loss' grounds having taken my deposit. Planning permission never discussed. 
Next came an independent installer. Split Daikin 14kW system, (which wouldn't require planning). £26k, which I wasn't prepared to pay. 
Then came British Gas at £14.6k for a 12kW Valliant. I accepted the quote last autumn, but they have now said that planning is required (over 0.6m3) and they won't install until I have got it. 
Recently another local supplier said £19k for a system over 0.6m3, but would install without planning leaving me with the risk of planning enforcement action. They also haven't followed up with a written quote. 

These quotes include VAT, but don't account for the £7.5k grant, which would come off. 
I'm stuck, and looking at getting a gas boiler. 

@pablito Yes to the first question. 

If it's over 0.6cu m they should refuse the LDC, however this limit may change upwards in the next months following a recent consultation so if you can hold on do (election may of course delay any change)

The ease or difficulty of getting express consent is entirely dependent on your planning authority.  Some have got consent without any problem.  My lpa imposes unachievable noise requirements and I have been forced to appeal a refusal.  Looking at other decisions they have made might give you an idea how yours responds.

 

11.2 kW Mitsubishi is within PD limits, and I think the dual fan lg is too, possibly also the Panasonic.  Unless you have a very large or very leaky house the Mitsubishi may well be sufficient.

@jamespa thank you, very helpful. I'm going to try to get a Heat Geek quote in the hope they can specify and supply a unit which falls within permitted development. 
When you say planning 'consent' this means planning permission? I don't think I can afford the time and cost of a full application. Bristol council has confirmed that they won't consider anything other than a formal application (lawful development or permission), as they are under special measures. 
Good to know that there are units which may work, but I am struggling to find someone to install one at an affordable price. 

Posted by: @pablito

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@jamespa thank you, very helpful. I'm going to try to get a Heat Geek quote in the hope they can specify and supply a unit which falls within permitted development. 
When you say planning 'consent' this means planning permission? I don't think I can afford the time and cost of a full application. Bristol council has confirmed that they won't consider anything other than a formal application (lawful development or permission), as they are under special measures. 
Good to know that there are units which may work, but I am struggling to find someone to install one at an affordable price. 

Sorry yes consent = permission = approval, terms used interchangeably.
A 'lawful development certificate' is nothing more than confirmation that the development falls within the (nationally specified) permitted development rules.  Your planning authority cannot grant a lawful development certificate if the development does not fall within the PD rules, and you would be wasting your money applying for one where the heat pump is oversized.  If the development does fall within the PD rules then a LDC isn't actually necessary, you can legally proceed anyway, however it can be useful when you sell the house particularly if the PD rules have changed subsequently. 
I would tell any prospective installer that you require the unit to fit within PD rules, and if they wont quote then look elsewhere.  Most installers confine themselves to a subset of all manufacturers and not all manufacturers do larger capacity pumps under 0.6cu m.
Would you like to tell us something about the house eg Age, construction, levels of insulation, floor area, EPC rating (or if you have it current fuel consumption).  From this its fairly easy to work out whether something like the 11.2kW Mitsubishi will do the job.  As you are in Bristol (fairly warm) it should unless the house is very large (>>200sq m) or very poorly insulated (eg solid walls throughout).  Far too many installers over-size HPs, most houses are around 8kW.  This leads to all sorts of problems of which tipping over the PD rules is just one!  Far too many installers overcharge and load the system with unnecessary kit too, you really do have to shop around.
 
 

Thanks for clarifying on the LDC. It would seem to me that a policy change to accommodate the larger units within permitted development would make a lot of sense. 
The house was constructed in 1905 with solid 18 inch stone walls. It is end of terrace, 4 bedroom, 175m2 and fully double glazed. It is currently services by a 12kW electric boiler, which we use sparingly due to cost.
I've completed my own MCS workbook and came to 12kW heat loss. I used the default values for parameters. The default MCS stone wall U value of 2.23 W/m2K is I suspect too high and has a big impact on the calculation. There's plenty of research (BRE, Historic Scotland...) showing that stone walls perform better than this. 
The Heat Geek website projects 10kW. We will never want the house at 21 degrees which the MCS works to, 16 to 18 is fine. I suspect an 11.2kW unit would be fine.