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I'm thinking you need to reduce the width of the bottom slate so it's no wider than the door then replace the membrane and cladding either side of the door so it extends lower down to match the rest of the build. But I'm not sure how to deal with the door frame. The waterproofing should be done behind the frame/cladding with ventilation at the bottom so the end grain isn't standing in water. I allowed about an inch on mine.

I just wrote back to the address on the letter and told them we wernt complete and gave them an estimate of when we would be. They sent back a revised letter.

The former can be easier. Typically each bathroom has a "stack" which is a vertical 110mm pipe in a corner. Pipes for the WC, shower and basin connect into this. The bottom of the stack under ground has a large radius vertical bend. The top of the stack either goes up through the roof and is vented to the outside or it terminates on the inside with an Air Admittance Valve. The AAV stops fast flowing water in the pipework sucking water out of the shower or WC trap. For rooms on the ground floor we sometimes run one 110mm pipe direct to the WC. 40/50mm pipes go out through a wall into a gulley. The challenge for this approach is getting the 110mm pipe for the WC in the right place. If it's too close to the wall the toilet seat might not stay up. If you want a walk in shower (or no plinth under it) special care and planning is needed when laying the slab.

Can't you rotate the supplied 90 so it points down and to the side. Then you just need another 90 degree bend.

That might be risky. Its not uncommon for planners to grant permission for a conversion but refuse a knock down and rebuild. I know that's not what you have done but it can invite a closer inspection from the planners. I would try asking the BCO for a letter confirming that "I have issued a certificate for a new build because you have carried out your conversion to new build standards". Something like that. If he won't give you such a letter I would write a cover letter to the VAT man explaining that its definitely a conversion as per your planning permission despite what the Building Control Completion Certificate states.

Ground anchors.. might be cheaper elsewhere? https://www.amazon.co.uk/Ground-Anchor-Heavy-Trampolines-Anchoring/dp/B0BRTTCZXW/ref=asc_df_B0BRTTCZXW/?tag=googshopuk-21&linkCode=df0&hvadid=658855764650&hvpos=&hvnetw=g&hvrand=15985417671135510135&hvpone=&hvptwo=&hvqmt=&hvdev=t&hvdvcmdl=&hvlocint=&hvlocphy=9045888&hvtargid=pla-2195966870085&psc=1&mcid=50cb5f09417a3df788aaba711c0c104a

If they are all the same it was probably original. I asked because if it was a more recent extension it might affect permitted development rights.

Has the house already had a rear extension since it was built?

I think the gutter above the red X might overflow in heavy rain. Our roof also has lot of dormers which collect rain from a large roof area and concentrate it down towards a short section of gutter between them. Its not so much the size and number of down pipes that seems to cause the problem but heavy flows run down the tile quickly and overshoot the gutter. Perhaps check the gutter is far enough away from the house to minimise how much water overshoots when its heavy. That said the occasional overflowing doesn't seem to create any problems for us.

We have several down pipes discharging onto pitched roofs. Instead of a straight down pipe I think it should terminate in a 45/135 bend that sends sideways at 45 degrees so that it runs down a wider area of the roof.

There are some fantastic 3D printed clocks..

I wonder if it's worth submitting your planning application with a request for the planners to add a condition requiring a "bat survey survey and satisfactory impact mitigattion plan if required" ? Might allow them to start processing the rest of the application

But can you put a heavy traditional 60-75mm thick screed on a suspended wood floor?

Does it warm your hands in winter 🙂

I'm sure Ive seen youtube reviews on different systems. Avoid strimming grass near fence posts. It can damage the thin pressure treated layer and cause rot at ground level.