Since 2020 it has been illegal for a landlord to rent out a home with an energy efficiency rating below an E.

But there are exemptions and gaps in the rules which the Duchy appears to exploit so that large chunks of Prince William’s £1.3bn property empire do not have to comply with modern standards. It leaves scores of his tenants, many of them elderly or on low incomes, stuck for years in properties the Government considers “sub-standard”.

Energy Performance Certificates or EPCs have been legally required every time a domestic property is put up for sale or rent since 2008. They use a colour-coded bar chart, similar to the ones on appliances like fridges, to rate a property from a green “A” to a red “G” – with “A” the most efficient and cheapest to run and G the coldest and most expensive to heat.

In 2018, the Minimum Energy Efficiency Standards or MEES regulations came into force for all new tenancies, and two years later they were extended to all tenancies, including those that had already started. Leading industry expert Andrew Parkin, director of Elmhurst Energy and chair of the trade body Property Energy Professionals Association, told us: “Under Minimum Energy Efficiency Standards regulations, any property that doesn’t achieve an E or better is in breach of those regulations unless they have a valid exemption. And there are a number of exemptions that can apply as long as evidence has been provided.”

There are six exemptions landlords can register for, and they include if the work costs more than £3,500, if all the possible improvements have been made and if the tenant refuses to agree to the work. Our research with Channel 4 Dispatches and Vera Productions shows the Duchy has applied just 10 times for exemptions and the most common reason, used four times, was that the work would cost more than £3,500 to complete.

Three times it argued that installing wall insulation would “negatively impact” its building, twice that “consent” to the work was refused and one it claimed exemption as all improvements had been made and yet the property had still not reached an E. We found 70 Duchy properties rated F and G with no exemption.

But there are a number of other gaps or possible loopholes in the regulations that mean even more Duchy tenants might not be covered. If a tenant moved in before 2008, there was no requirement for the landlord to get an EPC. If there is no EPC then there is no need for it to be an E. There are dozens of Duchy properties we found without EPCs.

EPCs expire after ten years, so for tenancies that started in 2008 and 2009, those EPCs wouldn’t have been valid by the time the MEES regulations came into force in 2020. So even though those EPCs are still online for the tenant and everyone to see, a landlord can continue to legally let an F or G rated property under those circumstances. We found two Duchy properties where that was the case.

It also appears that the Duchy carried out a significant number of “voluntary” EPCs in 2014 and 2015. Despite revealing that dozens of rental properties then owned by King Charles were rated F and G – and even though the EPCs were still valid when the MEES regulations came into force in 2020 – we understand the Duchy does not consider that the regulations applied.

Instead, its surveyors are analysing the portfolio and planning upgrades and this work is being carried out at its own pace without hitting the 2020 deadline. It means many of the Duchy tenants we spoke to have been living in properties the Government considers “sub-standard” for many years.

Lastly, it also appears that the Duchy believes that a number of its farmhouses that are rented as part of an agricultural tenancy are not covered by the MEES regulations. These are some of the oldest and least efficient Duchy homes and many have been occupied by the same families for generations. We have found around two dozen such farmhouses rated F and G.

Some lawyers argue that farmhouses rented under a farm business tenancy or the older Agricultural Holdings Act tenancies don’t have to have an E-rating or above because they are not domestic tenancies. Since this year, commercial EPCs also need to reach an E-rating, but some lawyers argue that farmhouses are not “non-domestic” and so are exempt from these rules too.

But we have found the Duchy’s own lawyers, Farrers & Co, arguing the opposite. Elizabeth Earle, Farrers’ “knowledge lawyer” for rural property, wrote on the firm’s website last year that: “Farmhouses are not referred to in the MEES regulations and this causes a problem.

“Although it might be tempting to try and exploit what may look like a loophole now, the more prudent position is to treat farmhouses as though they must comply with MEES. Doing so will be consistent with the overall intention of public policy and less likely to create a problem for the future.”

A Duchy of Cornwall spokesperson said: “We are a responsible Landlord committed to continuous improvement of its properties. We work closely with our tenants to actively address energy efficiency of properties across our portfolio while minimising the impact on residents.

“The Duchy of Cornwall is a private estate with a commercial imperative which we achieve alongside our commitment to restoring the natural environment and generating positive social impact for our communities. Prince William became Duke of Cornwall in September 2022 and since then has committed to an expansive transformation of the Duchy.

“This includes a significant investment to make the estate net zero by the end of 2032, as well as establishing targeted mental health support for our tenants and working with local partners to help tackle homelessness in Cornwall.”