Looking for a plain-English overview of Part L and Part F of the building regulations? Well, you’re in the right place.

What’s Happening?

The Government released major updates to Part L of the Building Regulations, Conservation of Fuel and Power and Part F, Ventilation, at the end of last year.

It also introduced a new Part O of the building regulations, which is designed to prevent over-heating in newbuild homes and other residential developments.

Changes to all three regulations come into force in England from June 15th

The links to each approved document are below. While we’ve summarised the key points, we’d recommend that you make yourself a strong coffee and sit down and take time to understand them.

And Why . . .

The changes that we’re seeing to Part L, Part F and the introduction of Part O, are interlinked and part of the Government’s strategy to decarbonise the UK economy and hit its net zero target by 2050.

Our homes leak masses of energy every year, accounting for around 14% of all emissions, so improving the energy efficiency of housing, including retrofit, is key.

The Future Homes and Building Standard (the new name for the Future Homes Standard since December 2021), is the Government’s roadmap for delivery - and tighter building regulations are how it’s going to do it.

The first big target is 2025, when newbuild homes will have to produce 75-80% fewer carbon emissions.

The targets, however, also apply to retrofit and home improvements as part of a target to cut emissions from housing as a whole by around 30%.

Parts L and Part F - a contradiction?

Well, the answer here is ‘no’ and ‘yes’ (or should that be the other way around?). Part L is about making new build and existing homes more energy efficient.

At the same time Part F says we need to make sure that there’s a constant level of background ventilation - which negatively impacts thermal performance.

New homes can be designed to deliver both. But things are more complicated, particularly in window and door retrofit, because we’re sealing up homes that weren’t meant to be sealed up.

That creates a raft of health problems - mould, damp, respiratory conditions – so it’s a trade-off. We need to raise the general level of thermal performance but maintain background ventilation, which is why trickle vents are here to stay!

See what the GGF has to say about Part L and Part F here

What are Part F and Part L requirements from June 15th?

Approved Document L

1What is Part L?

Part L sets standards for energy efficiency and carbon emissions of new and existing buildings. This means that it applies not only to new build properties but also home improvements, including windows and doors.

2Why has Part L been updated?

The Government has a target to hit net zero by 2050. To get there it needs to make homes more energy efficient, including those that we already live in.

3How is Part L changing?

From 15th June 2022, changes to Part L introduce new requirements for enhanced thermal performance in windows and doors as follows:


New build 

What is a notional value in new build?

Notional u-values apply to new build with windows and doors carrying a notional value of 1.2W/m2K. Notional u-values are a ‘recipe’ book for developers. Take windows with a u-value of 1.2W/m2K, add in roof insulation to ‘X’ and wall insulation to ‘Y’, and you’re going to meet requirements for Part L.

What you can also do as a developer is to decrease the notional u-value of one of those elements, as long as you increase it in another area, so the end result – the overarching energy efficiency of new homes – is still on target.

In practical terms this means that a developer could specify a window with a notional u-value of 1.3W/m2K, not the 1.2W/m2K notional u-value, and still comply with Part L of building regs.

What is a limiting value?

Notional u-values mean that developers have a little ‘wriggle room’ but there is still a line that they can’t cross, and that’s the ‘Limiting value’.
For windows and doors this is 1.6W/m2K. It doesn’t matter how ultra-energy efficient other elements of your development are. Windows and doors still have to be a minimum of 1.6W/m2K.

For a more detailed explanation click here.


If you are supplying new build projects you’ll also need to use air sealing tape around structural openings from June 15th prior to sealing.

Is there a period of grace before Part L comes into force?

Although there is a 12-month transition period for new build, new regulatory requirements under Part L come into force with immediate effect on the 15th June 2022. 

Do the changes to Part L apply to conservatories?

It’s fine. You can relax and take a breath. Conservatories are exempt under regulation 21 of the building regs as long as all of the following apply:

  1. The extension is at ground level
  2. The floor area doesn’t exceed 30 m2
  3. Glazing complies with Part K of the building regs
  4. Walls, doors or windows separating the extension are retained or, if removed, are separated with a wall door or window. 

Find out what the GGF is saying here:

How do we measure up?

Part L compliant windows and doors from Emplas


Have a question about Part L and F? Our technical support team is on hand to help. Call 01933 674880 or email [email protected]

Have a question about Part L and F?

Our technical support team is on hand to help. Call 01933 674880 or email [email protected]

Approved Document F 

Approved Document F

1What is Part F?

Part F sets out the requirement for ventilation in buildings, including new and existing homes, focussing on air quality and the replacement of old stale air with fresh air. It’s intrinsically connected to Part L and is designed to make sure that while we improve the energy efficiency of our homes, we balance it against a requirement for fresh air.

2How do you comply with Part F?

Approved Document F sets out three routes for compliance. This includes System 1, ‘background ventilation’ i.e. trickle vents; System 2, continuous mechanical extraction; System 3, mechanical ventilation with heat recovery and no background ventilators.

How will changes to Part F impact windows and doors?

New build

System 1 in Part F – background ventilation – is one of three options, which housebuilders can put in place to make sure habitable rooms are properly ventilated.

Windows and doors have a key part to play in doing this, and that means that unless another system is in place, new build windows will need to be supplied with trickle vents.


What does EA stand for in ventilation?

EA in ventilation stands for Equivalent Area. This is a measure of how much air passes through a vent as a measurement of airflow performance.

Part F compliance – replacement windows

Ok - this is where things have the potential to get a little bit more complicated but we’re going to start with basics and in that respect Part F replacement windows and doors are more straight forward because there’s no distinction between single and multi-storey properties - only the following requirement:

Does the update to Part F mean that you need to put trickle vents in replacement windows?

The blunt answer is ‘yes’ (well almost always) - even if they aren’t already being used. First-off where they are.

Replacing windows where trickle vents are already being used

If trickle vents are being used in pre-existing windows, Part F makes it clear that trickle vents will need to be used in replacement windows - and that they must provide an equal level of background ventilation. You should be able to see this printed on the old trickle vents. If you can’t you need to treat the windows as if they didn’t have trickle vents.


If you are installing replacement windows after June 15th 2022, make sure that you photograph the old windows to provide a record of what was there before and to evidence compliance with Part F.

Replacing windows which don’t have trickle vents

This is the big change under Part F. If the windows you’re replacing don’t have trickle vents you’re going to need to use them in the specification of new windows as outlined in Part F. In practical terms these will need to deliver 8,000mm2 EA if the windows are being installed in a habitable room or kitchen and 4,000mm2 EA in bathroom.

What if the homeowner doesn’t want trickle vents?

In short, it’s tough luck. As an industry we have to supply and install windows to building regulations and Part F makes it very clear that they need to be used in almost every case. Competent person schemes including FENSA and CERTASS have also made it very clear that indemnities and waivers offer no foundation for non-compliance - so in short, homeowners are going to need to accept that trickle vents are simply part of their windows from here on.

What if it’s not practical to fit trickle vents?

You’re occasionally going to come across jobs where it’s difficult to fit trickle vents. For example, what if the sash is too small to accommodate trickle vents - or at the very least to meet requirements in full. Part F makes reference to ‘Technical feasibility’. The GGF is interpreting this as what can be ‘easily achieved and convenient’. The suggestion is that if windows are too small to accommodate vents to meet the requirements in full, doing your best with what is ‘technically feasible’, will do the job.

Find out what the GGF is saying here:

Are there any exemptions to Part F at all? 

Ok – there are one or two exemptions to Part F (and Part L), mainly connected to listed building status and conservation areas, in which case exemptions may be applied for to not fully comply with the regulations in line with A7 to A13 of the Manual to the Building Regulations:

  1. Those listed in accordance with section 1 of the Planning (Listed Buildings and Conservation Areas) Act 1990.
  2. Those in a conservation area designated in accordance with section 69 of the Planning (Listed Buildings and Conservation Areas) Act 1990.
  3. Other historic buildings with a vapour-permeable construction that both absorbs moisture and readily allows moisture to evaporate. g., wattle and daub, natural stone etc.

Have a question about Part L and F? Our technical support team is on hand to help. Call 01933 674880 or email [email protected]

Our Part F solution

Trickle vents are here to stay. We’ve searched high and low and found what we believe is the most competitive, practical – and best looking option on the market – and that’s the Link Vent from Glazpart.

EMPGlazpart3D-HD 720p

This means that our trickle vent options available in 5,000, 4000, 2500 and 2000 EA, plus a choice of 20 different colours and finishes, providing a perfect match to our foils range.

We’ll let Glazpart explain:



Does the ordering process change? 

From June 15th when you order new windows and doors you’ll be prompted to specify trickle vents. We can’t tell you what to specify because we’re not carrying out the survey, so that responsibility sits with you – but we will remind you to do it and can offer you access to the industry’s leading trickle vent offer.


We’ll give the final word on Part F and L to our Sales director Emplas but in short, we’ve got you covered!

Approved Document O

1What is Part O?

Approved Document O is new and is singularly focussed on preventing the over-heating of homes in summer through solar gain. Like Part L and Part F of the building regs. it sits within the broader framework of the Future Homes and Building Standard. As it’s new build only, we’re not going to drill down into the detail here, but essentially it sets limits for the allowable area of glazing dependant on orientation. It also divides the UK into different areas of risk.

You can find out more here:

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