Worth looking at glazing cost comparatively. With that area of glazing you are going to have to use some fairly high specification 3G glazing, most probably with laminated glass for a lot of it. Sage glass is going to roughly double the cost at a guess, and will allow solar gain to be reduced to maybe 50 W/m² in hot, sunny, weather, a big improvement, but it will still mean having maybe 7.5 kW of active cooling for worst case days.
Using solar reflective glass will give about the same results as using Sage glass, for maybe half the cost, so will still need a fair bit of active cooling. The main disadvantage of using solar reflective glass is that you will also need more heating in winter, as there will be much less solar gain during the four months or so of the year when it might be useful. The cost of that additional heating may well equal the additional cost of Sage glass over solar reflective glass after a few years.
Realistically, I think you'll be looking at a glazing cost, including frames, just using conventional glass, with the required laminated panes, of around £450 to £500/m² at the budget end. You can get really budget windows for around £300/m², but not with the large areas of laminated glass you are almost certainly going to need, and not with the thermal performance that you will need to try and meet the regs requirements. I think you may really struggle with meeting the regs with 150m² of the very best glazing available, TBH, hence the earlier comment that doing a SAP assessment of the design now may well make a lot of sense, before you get too committed. If that shows that there is just too much heat loss through the glazing then you are going to need to have a re-think anyway, and better to do that before you've gone too far down the road at looking at different glazing options, perhaps.
Just a huge thank you for taking the trouble to reply in such detail. Lots to think about.
The last figures I spotted on here for sage glass were about £1k per m2 which would be way out of the ball park for me. But if I could find a reasonably priced supplier, then perhaps it could be back on the table.
Again, thank you.
To clarify the VAT postion, your conversion is either eligible for the VAT reclaim scheme or it's not. On the surface it looks as if it could well be. Read the notes in this link carefully to ensure that you qualify as the reclaim is at the end of the project so there is a lot of money at stake.
If you do qualify it works like this (all of this is in the link posted above but to summarise here):
Any materials you buy yourself (must be in your name) you pay 20% VAT and can reclaim ALL of the 20% back via the reclaim scheme
Any labour from a VAT registered trader is @ 5% and then you reclaim that 5% via the reclaim scheme
Any supply and fit arrangements by a VAT registered trader are @ 5% (the whole invoice) and then you reclaim that 5% via the reclaim scheme
If you use a non VAT registered trader for work they will need to charge you full VAT on materials as they don't have any way of reclaiming it themselves via a VAT return so if you are using non VAT registered traders try to supply the materials yourself (invoices in YOUR name) so that you can reclaim the VAT
Ultimately you should get all of the VAT back (on eligible items and work) if your conversion is eligible and it will put you in the same position as a new build. There are lots of other details in that thread about what is eligible and when things can be 'zero rated'. For new builds labour and supply & fit are zero rated so where you read 'zero rated' for a conversion replace that with '5% rated of which that 5% can be reclaimed at the end of the build'. Pretty much everything else is the same.
If you have any specific questions about VAT pop a reply on the other thread so that it keeps it all in one place.