Advice please - my builder installed a lintel that was too big

If that was specified on the drawing 150mm cavity and the builder chose to do it only 100mm there is nothing you can do as the building control won't sign it. Changing the lintel doesn't mean you follow the drawing which mean is out of scope. If the cavity was 100mm you just need another lintel or try to cut the existing one which again won't be acceptable.

Cut the lintel down

An engineer could inspect the installation and provide calculations/details showing that the lintel is adequately supported and that the eccentricity/overhang is acceptable. This is probably the first thing I’d investigate before touching the wall. Good luck.

No real alternative, unfortunately. If Building Control has rejected it, the correct lintel needs to be installed. As it’s the builder’s error, they should replace it at their own expense. I’d also ask Building Control if they’ll accept an engineer’s design, but in most cases replacing the lintel is the proper solution.

Is the 100mm cavity Kingspan?? If it’s 100mm wool then you can use a 50 mm insulated plaster boards

Build a stud wall with 50mm insulation across that wall and that will give u the 150 insulation required for sign off, if building control have told you that you need to knock it down then that is what u gonna have to do, and if u have paid in full you need to get your builder to do it but im guessing its the whole building

The ouside plate is the same your buiding inspector is rightly rejecting it as it is outside being able to step sorry but its a replace

Building control will not sign it off due to water ingress. I don't think the issue is the lintel as you can rectify this easily. You must have a residual cavity. But fear not as I have helped resolved this problem for customers before after rogue builders done this to them. step 1 is finding out the particulars of why it wont be accepted. if it is due to water ingress then from my understanding you have a few options. 1. render the exterior of the extension. 2. put cladding on the exterior of the extension. 3. apply storm dry to the exterior of the extension. all the above will stop water ingress and should satisfy building control. if they wont sign it off due to the build not meeting U values. then your only other options are as follows 1. Insulate the internal walls with a stud wall and Celotex with taped joints to make a VCL. 2. Use insulated plasterboard on the internal walls. 3. Insulate the exterior walls and render them. All of the above should help you with your issues. you are welcome to contact me if you have any issues or if you would like us to help resolve the issue with building control. Make sure you get approval with building control first before committing to a solution. I personally would also instruct your builder to down tools and get a new builder to take over the project.

Need to chage lintel and you put inside 50mm more celotex

Get calcs for new 100mm lintel and thermal calcs have them redesigned and signed off if possible, and present to building Inspector for review. We would normally expect to have to rebuild to correct drawings as passed by building control. If the builder built 100mm cavity below ground and slab is sitting on internal wall, you may have to cut slab to achieve 150mm cavity, Did the building inspector pass the foundations up to DPC, check your paperwork.

The builder should have been working to the spec of the drawing. If building control want it knocked down and rebuilt thats what you will need to do. Builders fault he is liable to fix said problem.