JohnMo

So how many joints that are not on corners? What's wrong with a joint or two, they are not seen, unless you start looking for them. Your gutters will be covering it anyway, so visually invisible. Too much other stuff going on for the eye to pick up on that detail. Step back a little and don't over think it. Or do a wooden facing board and have it wrapped in zinc while they are doing the roof? Then render up to the zinc

Something like this https://www.wolseley.co.uk/product/armaflex-evo-tube-selfseal-armaflex-evo-tube-selfseal-1-1-8-od-x-1-wt/

What difference does the shaded tree make to plant room location? As I mentioned previously you can make install super simple, and reduce the number of ducts required etc. by moving the MVHR You can make the original location work, but you need to detail how the walks are built. If stud walls, you need 2x layers of 18mm ply on the full wall to dampen any vibration and high density insulation. Heat pump or boiler? Where are they located?

The insulation thickness looks pretty poor. I would be looking for 25mm insulation minimum. You need to do pipe sizing based on pressure drop. So you need to know the kW required for the workshop. Then the flow rate can be calculated and pressure drops. But some comments on your proposed scheme I do something similar with a summer house. We did 28mm Hep2O, 25mm closed cell insulation, wrapped in foil tape (mice don't like it) then inserted in 110mm flex drainage pipe. Sealed ends as it came out of the ground with stainless mesh and spray foam. Cheap easy enough, everything available locally, so no transport costs.

Not strictly correct. With a service void, pipes can go in there, or with the actual roof structure. We have all hot water pipes and MVHR ducts within the roof structure and within the insulation. Green are the MVHR ducts, white hot water pipes.

All our rooms are vaulted. As @ProDave says you need a ridge beam then either plain timber, pozi rafter etc. Our pitched roof is 256mm pozi rafter, then below we have 2x 50mm battens, then 356mm of insulation full fill. Below we have airtight vapour barrier, a further 50mm battens and then plasterboard. Above we have 22mm sarking boards, vapour open membrane, and then slates direct to sarking boards.

The more boxes you try to tick the more confused any application, so more likely the planners say 'me don't understand' reject. If you are building to house rules minimum space requirements kick in, airtightness rules may make MVHR mandatory etc. If you are doing a caravan apply for a caravan, doesn't really matter if how you build it.

Point is why two heat pumps? When one will switch between the heating and cooling. Hitachi even have an auto settings that will automatically switch between the two settings based on what a intelligent thermostat reads.

You only need a single ASHP that does both. There is NO restrictions on cooling within BUS funding. There is via permitted development, but that is different. Just get a heat pump that does heat and cool out the box, Panasonic, Hitachi etc Open windows?

No sketch attached?

To the point that many said our house would be green from mould within six months.

Let's consider a few factors Air change rate. MVHR is in the range of 0.3 to 0.5 air changes per hour. Impact on room temp is almost zero. Two windows open on different sides of the house could 6 to 8 ACH. Look at the performance chart of cooling batteries, apply your flow rate and the cooling water flow temp and see the resulting kW heat removed. It is small. If you have cooling water, spend money on a fan coil or two instead of radiators or as well as UFH. If you have UFH just push the water through the floor, it's free cooling with solar PV.

Your issue is water tightness - keeping water out the roof structure.

Don't waste you money - read the many threads on here. At those prices I would be doing it myself and did. Your layout lends itself to a nice simple cascade system. First MVHR move away from bedrooms and install in cycle store. I would flow into dining and into office from there only. Dining flow will be most the house volume flow out of two outlets. Extract from all wet rooms and plant room. All other areas will be ventilated via cross flow or transfer flow.

Unless additional mitigating measures are taken any building can leak. Air tightness is all about details, that are added at the design stage normally. Brick and block benefit from wet plaster as it seals all the small gaps everywhere in the cement joints. Dob and dab done very well is ok also. Window and door design and fitting. Floor to wall and wall to ceiling junctions can leak a lot. How sockets and light fittings are done all need thought. Lots of small gaps equal a big leak.

2m X 2m is f...ing huge are you certain you need it that big. Price will be pretty huge also, by the time the huge support structure is finished, so hope you have deep pockets. Our sky lights came from Roof Maker, take a look at their website. The sky light will sit on upstands, the upstand will sit above the roof structure. So will be raised above the roof line, not flush.

Nothing like like having a good think about it. 1. Smaller maybe not, but if visually different you may need amendments or new planning. 2. Most are, but if you are uses your structural engineer they will need to handover. 3.? Interference not sure what you mean. Suspect these companies if you choose a plan that already exists they will not let you interfere much apart from some finishing details.

You could make holes in the wall and add plates, or just get the chippy to set door frame slightly higher in the wall. No skimming of doors, zero cost adder.

No issue blanking stuff off etc. I would make sure you have cone filters in the extract points and replace on a regular basis to keep ducts clean.

Because MVHR supplies air to dry rooms and extracts from wet rooms. Without the undercut air stops flowing when you close doors. Don't fret about acoustics the under cut makes zero difference. No You don't with MVHR but you need to allow cross ventilation with other classes of ventilation such MEV. But then you add humidity activated trickle vents to minimise ventilation when it's not needed. All houses need ventilation just a matter of how it’s done.

You use the power in watts to size each room radiator, so room 1 needs 618W output from radiator. Now you need to convert the radiator size from manufacturer data to your chosen flow temp. There is a calculation on here somewhere so do Google search on buildhub. You need to look at the pipe sizes then as most of the micro bore may need replacing(?). Did they install a buffer?

I hired someone as a labourer. Best money I spent. You have to get up to collect them at a fixed time. You work a proper 8 hour day. You drop them off at home again. You generally just tidy at the weekend so you are fresh on the Monday. You have a really productive week. You stop for a breaks and lunch. They are a sounding board for stuff. Doesn't matter if they know the answer, you have a chance to speak through stuff and it gets resolved quickly. If you fall, have an accident etc on your own you are stuffed, so a big safety bonus with someone else there.

We've got a Sarnafil mechanically fixed and welded membrane. Great stuff, easy to do upstands etc. Comes with 10 year manufacturer guarantee, only qualified installers get to install.

I do about (each way) 28mm pipe 2m above ground and then about 6m through an insulated shed and then about 5m underground. All done in 25mm closed cell insulation, below ground each pipe is in 110mm duct about 600mm below ground to the house. Plus I have 15mm pipe with 19mm insulation, wrapped in foil tape (keep rodents off) from insulated shed to summer house above ground a run each way of 16m It's do able and I don't see much effect. Decent flow rates and velocity from heat pump helps.

I would have done OSB, membrane, vertical battens and horizonal battens, corrugated. But with corrugated roof do the battens bring anything to the party? Do you need them at all.