Temp

Planning Permission doesn't help him. PP only means its acceptable as far as planning policies are concerned. Its quite possible you can get PP but not be able to build for a dozen other reasons. These days drainage can be a show stopper eg Soakaways don't work on clay and the Water Co won't allow you to put surface water into the sewer. Unfortunately there is no penalty for not complying with the PWA and nobody to enforce compliance. It would be down to you to go to court and try and get an injunction which is difficult and expensive. As I understand it judges frequently allow overhangs to continue and award you tiny damages. Once walls have been started it gets expensive for your neighbour to fix the problem so chances are they won't do anything if walls are already going up. Perhaps too late for you but it's always best to discuss plans with a neighbour at the outset. Hint you are likely to build your own extension so you need to figure out a solution that works for both of you before the walls are set out.

Perhaps check the planning file for your house/estate. Sometimes the planners want a sample submitted. There might be a letter from the builder telling the planners what he's going to use, or take a drive around and see if you can find some other house that has an extension an ask them if they worked it out.

Did you dig a test pit to check out the foundation depth of the main house?

+1 Soakaways are meant to be a minimum distance of 5m from the house.

Possibly Corian ? If it's that it's quite expensive, lots more than Travertine.

Planning Permission is required if you don't deal with rainwater run off on site. So if not permeable you will likely need a linear drain to a soakaway to stop water running onto the road/pavement. In many areas you can't put surface water into the sewer.

Remember the opening part must be a minimum height above the floor to meet Building Regs (fall from height). Think it's 800mm but check.

Is the timber treated? Can be difficult to glue well to that. Otherwise Id use a polyeurathane wood glue which foams up to fill any voids.

Google "for joist repair plates" or "Bower beams" https://www.permagard.co.uk/bower-beams-large?gad_source=1&gad_campaignid=20104758639&gbraid=0AAAAAD6l7u6eYbKpQcQBXyc85Smq5MoS8&gclid=CjwKCAjw87XBBhBIEiwAxP3_Axxih9ZPj7neGI8nFscH233n-kk6VjP3GPKyrL0vkQGshG4fCttAOhoCbzMQAvD_BwE https://www.preservationshop.co.uk/product-category/timber-treatment/timber-repair-systems/

Can you add a tie(s) at floor level?

All I can see is something wrong with the very top row of tiles on the lefthand side.

The planning system has a few things like that. A wind farm can argue they won't make significant noise to get permission then object to new houses nearby on the grounds of noise.

I would ask an expert to comment on the enforceability of the covenant as it's not uncommon for the wording to be faulty or for it to expire after a sale. If said expert says it's still valid you may have to approach the council. That could go either way. They might refuse or agree to remove it but for a fee. They might insist they will only remove it if you build an affordable house or who knows. If it was discovered after the house is built a court might decide you should split the profit that their agreement provides. It would be hard to sell with a covenant on it.

Did they mean it wasn't on the address database? You can fix that.

I noticed on page 59 of the install manual this diagram has a "buffer" zone valve output. Is your valve connected to that? or to the DHW output?