The change in calculation methodology (which mandated  BS EN 12831-1:2017 as opposed to the simpler calculation) were first published early in 2025 and became mandatory on 2nd Jun 2025.  There is a useful but not authoritative summary here and the definitive document is here

If the contractors have not complied with the new MIS-3005 then the installation does not meet MCS standards.  If the difference that the calculation makes matters to you then I would definitely insist in writing and, if they cant explain their reasoning, refuse to pay for the survey (if it was chargeable), look elsewhere and consider copying their response to MCS as evidence that they are not complying with their certification requirements. 

If it doesn't matter to you then you could take the risk, which is likely rather small given the poor record that MCS has of enforcement, but why should you unless you have very good reason to trust these particular installers.  If they are 'unhelpful' at this stage what will they be like later?

The new methodology is said to result in more realistic ACH figures, whereas the previous one frequently overestimated.

Posted by: @jamespa

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If it doesn't matter to you then you could take the risk, which is likely rather small given the poor record that MCS has of enforcement, but why should you unless you have very good reason to trust these particular installers.  If they are 'unhelpful' at this stage what will they be like later?

JamesPa So the risk of MCS picking this up is small, which is good to hear.

The prime contractor has a good track record, so I am I little surprised at them not complying with the regulations.  in terms of my specific requirements (personal and technical) this company ticks all the boxes.  They are in fact my only option.

Posted by: @trebor12345

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Posted by: @jamespa

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If it doesn't matter to you then you could take the risk, which is likely rather small given the poor record that MCS has of enforcement, but why should you unless you have very good reason to trust these particular installers.  If they are 'unhelpful' at this stage what will they be like later?

JamesPa So the risk of MCS picking this up is small, which is good to hear.

The prime contractor has a good track record, so I am I little surprised at them not complying with the regulations.  in terms of my specific requirements (personal and technical) this company ticks all the boxes.  They are in fact my only option.

 

I think its small, but off course it could happen so please dont rely on it.  They do a sample check of some kind on installers and my system was checked as part of this process.  However I think you can opt out (I had to give permission).  Obviously the installer is unlikely to say anything.  Are you claiming BUS? I guess not and if not then no risk there!

My guess is they havent yet updated their software with the new regs!

Posted by: @jamespa

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Are you claiming BUS? I guess not and if not then no risk there!

You are correct no BUS.  Are you saying that the MCS certification is only an issue if there was a BUS?

@trebor12345, in my opinion the MCS certificate is functionally worthless to a homeowner beyond unlocking the £7.5k BUS grant. That’s it. Strip everything else away and that’s the only tangible benefit.

MCS likes to present itself as a quality gatekeeper and a consumer protection scheme, but it’s neither. When installations go wrong (and we see plenty of catastrophic ones) MCS and its associated consumer codes are nowhere to be seen in any meaningful sense. Endless forms, endless process, zero accountability, zero outcomes. I’ve yet to witness MCS actually step in, take control of a situation and force a bad installer to properly put things right.

The truth is that MCS exists to protect the scheme, not the homeowner. It polices paperwork, not workmanship. You can be MCS certified and still deliver an utterly incompetent, badly designed, badly commissioned system, and there are countless examples of exactly that.

Homeowners are paying for that certificate either directly or through an umbrella scheme, so if you go with an MCS installer that registers the job with MCS you will be paying for that. 

Posted by: @trebor12345

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Posted by: @jamespa

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Are you claiming BUS? I guess not and if not then no risk there!

You are correct no BUS.  Are you saying that the MCS certification is only an issue if there was a BUS?

 

Since May 2025 it no longer a requirement in England to benefit from permitted development rights (ie planning consent without having to make a planning application), the only MCS related requirement for planning is to meet the MCS noise spec.  Not sure about Scotland, Wales and Northern Ireland, they may be behind England in changing the legislation.

There is talk that some solicitors/conveyancers might ask for it when you move house, but I would think that its unlikely to be a deal breaker unless the person in question is really risk averse and doesn't much want the house.

Other than that I don't think that absence of MCS is an issue.