In 2013 we had our newly bought outdated house completely renovated/extended.

My wife was pregnant at the time and we made a rushed decision of either waiting until our children were a little older to carry out the huge amount of works or get a company to do it there and then without thoroughly researching…
Through local recommendations, we found a company that gave us a good price and were able to start within the 2/3 months mark of the initial meeting.

It took the company around 4 months to complete the job (we naively let the owner project manage everything from submitting plans to meeting with the building inspectors) however very soon after the works were complete and we received our building reg certs, we noticed foul drain smells coming out from our newly extended kitchen/diner floor and newly created bathroom (we even have rats continuously crawling under the kitchen….)

Long story short, we had the company out many times to try and rectify the issue but were unsuccessful (We even had drain CCTV performed twice at our own costs which showed no issues).
From August 2013 until December 2015, I had continuously tried to speak and email the owner to rectify the issue but he basically said in the end he had no other options and ways to resolve the issue.

After my last email in December 2015, I decided I cannot expect a change around from the owner and I now had to cut my loses and get a company in to rectify this problem.

Since then we have had a few construction companies in and they have advised us the flooring was incorrectly laid, issues with ventilation and the drain smells issue seems as if there is either an uncapped old unused drain or/and issues with the drain pipes.

A couple of the companies said they were even surprised that building control had signed this job off (especially when it came to the ventilation issue).

I am now thinking maybe I should find out what went on with building control as I never got to see them during their visits (both my wife and I was working) and we have heard the company owner and inspector are very good friends and may of not even made any visits that required thorough inspection…

Has anyone else been in this situation before and how would you tackle it?

If be speaking to building control – there must be records.
Sounds like the floor needs pulling up and relaying properly. I’d also be speaking to your insurance company as you may be covered.

Tricky situation. Sounds like a dodgy private building inspector. If they are matey with the builder there’s a chance they never even inspected. You may have some recourse if the builder is registered with one of the schemes but it’ll be a pain. Moving on and dealing with the issues is the way to go. Ultimately complying with building regs is the responsibility of the home owner. The drains is the main issue. I’m surprised the cctv inspection found nothing. They picked up my neighbours uncapped branch when she had the cowboys in and resultant rats. That needs more investigation. Ventilation should be fixable retrospectively

edit. Where is the drains smell? Inside or out? If inside it could be a trap that is losing water due to too much vacuum. Maybe fixable with an aav. Just a though.

Was the extension built over existing drainage? For example, was there a man hole cover that was built over?
As you have a suspended floor (from your description) It seems most likely to me that something was either built over and not sealed properly. Building over man holes isn’t necessarily a problem as long the the cover is an airtight one. Or, there is a broken or disjointed pipe somewhere.
It does not take much of a leak to allow sewer gasses to seep into living spaces.
Also, you need to check where the vent stack for the system is located. Unfortunately, for you, you need an expert to figure out what’s been done and how best to resolve.
edit: The rat problem may or may not be related.

I’d get an independent report done outlining exactly what is wrong, place it in writing then to the builders giving them 14 days to respond and for them to confirm when they are going to rectify the work, if they don’t respond then Smalls Claims Court?