@kubak, 

• I assume you're getting planning permission for your extension?  If so and you want an ASHP you might want to include it.  Otherwise check out the permitted development requirements for ASHPs for Scotland; there are some restrictions
• @Julianc has installed an ASHP with microbore, maybe he can advise?  I would have thought ufh would be a lot more expensive than replacing the microbore pipes (if you need to). Even if you don't have ufh you might have to have bigger radiators.
• I don't know about Clean Homes Grant but I think Scotland may have its own schemes not available to England? 
• There is nothing to stop you doing the work to improve insulation during or even after the ASHP installation and commissioning but you'll need it done before you apply for RHI

What's your current system?  Gas or oil?  And what is the current heating and HW numbers from your EPC?

Hi Kuba

I think it’s all about getting several quotes and approaches. 8mm pipes are small to the radiators, but do they run all the way back to the boiler at 8mm? Or are the “main lines” 35mm or 28mm?  That’s eh here a good ASHP engineer would be able to advise. As KevM says, we have just had an ASHP fitted. 10mm pipes to the radiators. But early days - just over two weeks of running so no data. UFH is best, but my wife said no to the upheaval. 
The design of my system places a buffer tank between the ASHP and the radiator system. This alloythe ASHP to run at its pace / temp. And then we have a big, variable pump (nearly £500!) to push the water around the radiators. I had watched videos and heard concerns about noise from a higher pump, but we have not experienced that so far. The mantra for ASHPs is however lower temp and get the flow right. 

Thanks both for your thoughts!

@kev-m

• We're still deciding on an architect, but most agreed that our extension will fit within permitted development. The permitted development rights for ASHPs in Scotland have some strange clauses, e.g. we're only allowed to install one heat pump, and it has to be removed immediately if it no longer works. Doesn't affect us, but I'm struggling to imagine situations where these would apply! 
• The Clean Homes Grant appears to be replacing RHI from next year, UK-wide. I'm not sure if it has been approved yet, but this is what I found on it: https://www.homeheatingguide.co.uk/grants-schemes/clean-heat-grants  You're right that Scotland might have different funding! There are definitely 0% interest loans than we can apply for to cover installation, but it also might be worth waiting to see what Holyrood decides after yesterday's election. 
• We've currently got a 24kW gas combi boiler. 
• Is it the heat and hot water demand numbers that you're looking for from our EPC? We're at 8998 kWh/year for heating and 2108 for hot water, with a predicted impact of -1934 once we install cavity wall insulation. In reality, our gas consumption over the last year was 13994kWh, but I suspect this was largely due to the cold winter and lockdown!

@julianc Downstairs they're 8mm all the way to the radiators. Upstairs we have a wider main line.

• How much space does your system take up inside, including the buffer tank? Might have to consider a larger utility!
• Do you know what the dB rating is for your pump? 

I thought I'd give an update on where we are, and maybe also give this thread a little bump in case anyone has any other thoughts. 

Unfortunately we have had quite a large insurance claim due to escape of water, so plans for extending and heat pumps have been delayed a bit. On the plus side, the house is gutted, so once nerves settled, we took the opportunity to upgrade our plumbing. Our plumber discovered some pretty horrific things that the previous owners had done with the microbore, but now we have nice, new 15mm piping going to all of our radiators.

Getting a survey done next week to see if we can install cavity wall insulation - our house is pretty exposed to wind-driven rain, so we'll have to wait and see what they say about that. Not sure a heat pump would be viable without cavity wall insulation, I'd have to re-run the numbers.

After speaking to a few different installers, I have a pretty good idea of what needs to be done, so now I can weigh up the "should or shouldn't I" question. In theory it seems like a great idea, but what I'm lacking is evidence for houses of a similar style to ours. If anyone has any stories about retrofits to a 1970s semi-detached, I'd love to hear them.