Mr Punter

I thought site access was bad, so smaller units would be easier to move. Blockwork is good as it will not rot. You could do a single skin of 140mm medium density blockwork, left fair faced internally with external EPS insulation and thin coat render on the outside. You could DIY the insulation if you wanted to save money. A pitched roof is likely to be lower risk.

Backdrops not required. Given that this is only a gym I would want to reduce the cost by 40%. EPS insulation is fine under the slab and if you DIY will save 70%. You may get a more economical slab design. The £4,700 for the trench and buried plastic seems steep. With the prelims, because they are a percentage these should come down as well. Just use this as an initial quote and look at savings that can be made. Because it is a fair sized slab you will need someone competent to pour and finish the concrete. Regarding the build method, why not blockwork cavity walls with insulation? Does not need to be passive standard, just normal batts or partial fill pur. If you are keen to muck in you could load out the blocks for your brickie. You could have the brickie and his mate do a day a week and they would do it in no time.

Your prelims are for managing the job, RAMS, PPE etc. May also be for overheads costs and profit. Skips - for concrete pump washout? Offcuts from insulation. Packaging. Pallets. Pipe and duct offcuts. Tree roots. Not sure how deep the foul is going but it doesn't sound crazy. Can you exclude any of these from their works and do yourself? At least now you have prices and quants you can compare to others / look at SPONS.

Keep Compriband in the fridge if you are using it in warmer months.

Tarkett, Polyflor and several others do it. Non slip and no leaks. It can look a bit institutional if you do not choose wisely.

Is this in your dungeon place or the dosshouse Luxury HMO?

Some wetroom vinyl flooring then? It can be stuck down, sealed at edges and will not crack or lift.

What is the wall construction? If is is cavity, there should be a cavity tray / DPC above the window lintel sloping to the outside leaf with weep holes either end to let water drain out.

Take up the tiles and fit LVT. Much nicer underfoot. Looks fantastic. You can make up any height difference easily.

Could you kiss goodbye to the tiled floor and fit a bespoke shower tray? https://www.versital.co.uk/bathroom-products-2/bespoke-shower-trays/

The quality films like 3M Prestige are very expensive and require specialist fitting and edge sealing. £80 per m2 as a budget cost.

No, it needs to be zero rated. The labour element will be OK for him as he will not need to pay the VAT or charge it. He will be able to reclaim the VAT on the materials. If he does not have the cashflow to do that you could pay for stuff like concrete directly, although if he has accounts with suppliers he should be able to bill you before he pays them.

Yes it is just outside the CA with no TPO. The branches have to come off to enable construction.

Put the dehumidifiers x2 downstairs. They also add some heat. Have trickle vents in the downstairs windows and extracts upstairs. The gap under doors works well to allow easy air flow.

You could breed with @ToughButterCup and your spawn may have 10 digits!

I have recently bought a plot with an overhanging tree from a neighbour. I will just pay to have the overhanging branches cut back to the boundary.

Is the high humidity throughout the building and throughout the day / year? Have you looked at a dehumidifier instead of increasing ventilation?

Could you have an air leakage test done cheaply? You could then work out where most of the uncontrolled leakage is. The extract rates seem rather low. Can you increase them? Maybe double or triple. In the plant room you could just have a light switch or pull cord fan. I doubt if heat recovery will make much difference to your bills. What is you heating fuel?

@Onoff gets the prize for most geographically distant British Mainland cultural insults in a thread page.

curiouser and curiouser...

No just looking at the guidance here where the reason the lower one is not acceptable is stated as it extends more than 6 metres beyond the rear wall. No mention of "wraparound" in the document.

It is different in Scotland where they have proper weather.

The existing slates may be worth a few quid as reclaims.

None of the the guidance makes any mention of "wraparound", so it is not a relevant definition or description. This is a rear extension. If the extension is 6m or less from the rear wall it is OK. He may need to create a slight dog leg on the kitchen side (if the pantry is currently the outside wall an the distance to the proposed new rear wall is >6.0m) but it looks to otherwise comply.

You don't see that down here but we don't get the weather you do. I like local vernacular building methods. He didn't scrimp on the slate and a halves / doubles either.