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The mortgage needs to be secured on the land. They can't do that as it's effectively undefined until you split it. You don't want it secured on the whole plot either, only on the bit where the new house will be. This is all normal and will apply to all lenders

There are several low profile solutions for UFH but unless you have at least 80mm of insulation below the concrete I would avoid them. We have 80mm and wish I had fitted more. If you really want UFH think about taking up the floor.

More insulation and avoid wood windows. Had enough of painting.

I agree with others.. 150-200 mm PIR and UFH in screed of 70-80mm Avoid Anhydride (spelling?) screed because it's hard to tile onto. Form expansion gaps at doorways don't screed right through as it will crack. The expansion gaps is just a deliberate gap so you ensure it's straight not curving into one room or the other. If carpeting go for special low TOG underlay and keep total TOG to 2.0 or less. Stone and tile is great with UFH.

Spiders and bees sometimes block overflow pipes

Could easily fill wood frame with offcuts of insulation. I didn't bother as all showers on first floor.

Some top access traps have a higher waste than others.

I've done two or three shower trays on beam and block floors with insulation, UFH and screed. I like the waste pipe to enter the area under the shower as far as possible from the trap so the trap can move a bit in all directions. Not essential but it makes alignment a tad less critical. Then I build a timber frame as per option 2. Normally treated 2*4 cut down if necessary. At least 18mm WPB on top. I cut a clearance hole in the WPB so the bottom part of the top access trap can be moved about a bit in all directions including vertically so it can be pulled up against the underside of the tray by the top part. This is why I like some unsupported length of waste pipe. I do a dry run then mortar the tray down. I Used flexible tile adhesive last time as I had some already for tiling. Used a notched trowel. I screw in the top part of the trap while still soft. Later removed and refitted with sealer. Two of the trays are virtually at FFL but I allow a bit of clearance so the bottom door seal doesn't rub on the tiles when you open the door.

I recommend producing a street scene drawing/sketch showing your proposal and the two houses either side. Our architect did this using pen and a single water colour (green). This allowed a certain artistic licence and harmonised the different designs to certain extent.

Perhaps something like this.. https://ocldirect.co.uk/natural-stone-grey-tumbled-block-paving-200-x-100mm-per-m2 £52 x 40 = £2080 But don't underestimate the cost of mortar for pointing.

When my Aunt died we had to extend her lease before selling her flat. Freeholder delayed and delayed forcing us to take them to court which cost even more time. Eventually the council slapped us with double council tax because the property was empty. We finally completed on the sale in June, stopped the Direct Debit and wrote to the council to notify them. Months later they still haven't responded stating they have a three month backlog of post ! This is now delaying us winding up her estate, most of which is going to charities.

I found this guide some time ago. Haven't checked if it's still current though... https://electrical.theiet.org/media/1695/electrical-installations-outdoors-a-supply-to-a-detached-outbuilding.pdf I think mine was done as per figure 3 eg.. MCB in the house CU. SWA cable to a "Garage CU" in the outbuilding. Garage CU contains an RCD and two MCB (lighting and power) Earth provided at the outbuilding via a rod.

I've hired a few towers over the years. I probably should buy one. A pet gripe is that some companies work on the platform height, some the hand rail height and some the working height. The latter is about 6ft above the platform height. It can be hard to find on their website. You need to check. Last one I hired via https://www.lakeside-hire.co.uk they subcontract it to local hire companies outside their area.

Why 3 batteries? Does the system need 12V and step it up to 24V?

I agree it's most likely an old chimney. You can see remains of the flashing at the top.

Ah sorry my bad.

It's measured from the ground adjacent to the building. If the ground slopes it's from the highest part adjacent to the building. You might get away with raising the ground slightly but if it's obvious that's what you have done the increase will likely be discounted.

It's amazing what some will complain about. They might just complain it needed planning permission.

In some areas planning officers are part time, only working 3 days a week. I believe they work the other 2 days at another county.

If you fit clear glass and the neighbour complains it's likely the planning office would require you to apply for retrospective planning permission or fit obscure glass. They do this just to shut the neighbour up and or for the planning fee.

Tape the joints in the foil covered insulation with metal foil tape to form vapour barrier.

One option would be to fit clear glass and some sort of obscure film. At least then if the neighbours move you can peal it off.

Unfortunately that's not what the tech guidance says. It doesn't need to be fixed shut but it does need to be obscured.

Source https://www.gov.uk/government/publications/permitted-development-rights-for-householders-technical-guidance So unfortunately you are correct, it must be obscured to level 3. How well do you get on with the neighbours? If they are happy I would just risk installing clear glass if that's what you want. The planners are very unlikely to come check once the original build is complete.

I used flexible tile adhesive. Seems OK. However some trays aren't very flat on the bottom or strong enough. The mortar base is intended to solve both issues.