6 Rules for Adding Modern Glass Extensions to Old Buildings

If you were to ask us what we love about the UK, our answer – aside from the Beatles and a good cup of tea – would have to be the sheer volume and depth of history that is evident everywhere you look. Saxon churches, Norman castles, Tudor manor houses and Victorian warehouses punctuate our landscapes, telling tales of the past with their weathered stone and intricate woodwork.

To those in possession of one such historical property, firstly let us say congratulations – you are a custodian of time, a keeper of centuries-old stories, and a guardian of architectural heritage. That being said, old buildings are not perfect – often far from it (and that’s what makes them special). As a result, your property might not be quite as bright as you would like it to be, quite as big as you would like it to be, or a combination of the two. If you can identify with either of these wishes, a modern glass extension to your old building might be the answer.

An extension on a period property can either look great, enhancing the original features, or it can stick out like a sore thumb, detracting from the charm and character of the building to which it is attached. In this blog post, we have outlined six rules for adding modern glass extensions to old buildings, ensuring that you fit firmly into the former category, rather than the latter. Some of these will be in direct relation to planning laws, others are included simply to make your life a little bit easier.

Rule #1: Get Listed Building Consent if needed

According to Historic England, the country is home to approximately 330,000 listed buildings. 2.5% of those are Grade I (think Buckingham Palace and Tower Bridge), 5.8% are Grade II* (Battersea Powerstation or Downe House), and the remaining 91.7% are Grade II (Abbey Road Studios). 

If you own or intend to carry out work on a listed building, it is most probably a Grade II building. Being listed doesn’t automatically mean that you can’t make changes or add a glass extension. It doesn’t even necessarily mean that it will be hard to make those changes, but it does signify that the character of the building is important and will – at the very least – be a consideration that needs to be taken into account.

If, for example, your rustic cottage has charming windows with wooden frames – the sort that would have adorned the property 200 years ago – it is unlikely that you will be able to replace them with cheaper UPVC windows (in fact, you almost certainly won’t be able to). But a glass extension that is considered to be visually unobtrusive is another matter – because it does not alter the fundamental character of the original building, you will most likely be able to go ahead with it.

But getting the go ahead can only happen once you have acquired Listed Building Consent (alongside the general Planning Permission which can be required irrespective of a building’s age).

We have written previously about getting approved for Listed Building Consent, but your first step will be to speak to your local Conservation Officer. Every listed building has a different character and different considerations, so it would be wrong for us to provide broad brushstroke advice. As part of your proposal, it is important to demonstrate that you have taken the character and history of the building into account. As we have already mentioned, a glass extension is the ideal addition to most listed buildings because it does not significantly alter the original structure, making it far easier to demonstrate that you have considered its existing character.

If your building is old, but is not listed, make sure to speak to your Local Planning Officer. Typically, planning permission for extensions is only required in the following instances:

• If it covers more than half of the area around your property;
• If it is built onto the front or side of your property so is visible from the road;
• If it is higher than the highest point of the building’s roof;
• If it is more than 3 meters beyond the rear wall of an attached house, or 4 meters beyond the rear wall of a detached house;
• If it uses materials that are different from your existing house; or
• If it sits within a designated area, or falls under an Article 4 Direction.

If you are in any doubt over whether your extension requires planning permission, speak to your Local Planning Officer.

Rule #2: Use high-quality materials

You will know better than almost anyone else that the construction industry is filled with cheaper alternatives. Instead of sawn limestone, you could go for Indian sandstone for your patio, instead of metal, you could choose plastic, and instead of bespoke, you could choose boilerplate.

Nine times out of 10, nobody will notice if you went for the cheaper alternative; often it’s hidden, or you would have to get up close and personal before the difference became apparent. With a glass extension, however, opting for the lesser of two options could potentially cause you problems – both structurally and in terms of regulations (as we have already seen). For example, using lower-grade glass might save you some money upfront, but it could compromise the durability and safety of the extension. Likewise, if the frames or glazing infrastructure degrades or warps, you could face problems with leaks, drafts and even structural issues that require costly repairs.

Our advice would be to seek professional advice, ideally specialists who have been designing stylish for over 30 years and can help you to bring your design to life without falling foul of structural or regulation requirements. We would be happy to offer that expertise.

Rule #3 Ensure compliance with building regulations

As anyone who has been part of a building, renovation or extension project will know, the list of UK Building Regulations is very long, continuing to grow as new widespread concerns develop and need to be addressed. As a result, many of the 19 parts will not apply to your modern glass extension on an old building because of the type of structure that it is. That being said, there are still several important regulations that your extension must adhere to*.

(*Failure to comply with building regulations could lead to a demolition order, and regular failure could lead to fines and legal action being taken.)

The key parts of UK Building Regulations that you will need to consider include:

Part A – Structure

Your glass extension must be structurally sound. That means its foundations, glass panels, and supporting frames are capable of bearing their collective weight, as well as withstanding the changeable UK weather without being damaged or falling down. 

Part B – Fire safety

As well as structurally sound, the materials used to build your extension must also be able to resist fire. If you are choosing a modern glazed extension, you are highly likely to fulfill this requirement quite easily because glass is not flammable.

Part K – Protection from falling, collision and impact

For a glass extension such as this, Part K is perhaps the most important. The glazing you use must be robust enough to withstand impact, and any openings such as doors, must be positioned in a way that will not cause breakages. Part K is also relevant for things like suspended glass floors, requiring not just additional thickness, but lamination to prevent injury in the event of breakage.

Part L – Conservation of fuel and power

Whereas the original building to which the new structure is attached may be drafty and quite cold in winter, your extension must minimise heat loss and gain through the inclusion of energy-efficient glazing and the appropriate amount of insulation.

Rule #4: Listen to the original structure

This advice might sound a little bit New Age, but it is a really important step – often taken subconsciously – when adding your modern glass extension to an old building. By “listening” to the original structure, we mean paying close attention to the design, materials, and character of the existing building. Understanding these elements will help you create an extension that feels like a natural continuation of the original architecture, rather than an afterthought. Many of our clients choose a frameless design, minimising the visible structure, but if the building was built using sandstone, you might choose to incorporate light-coloured wooden elements that enhance the existing materials. The key is to be as unobtrusive as possible, letting the character of the original building shine through. Your extension should be a spotlight – it shouldn’t steal the show.

Rule #5: Incorporate positive landscaping

As we have already mentioned, a good glass extension is unobtrusive, unassuming, subtle, enhancing the building and its surrounding area. That surrounding area can be made significantly better through the planting of a beautiful garden. Perhaps you might like to shroud the glass box extension with a ribbon of native hedgeplants like hawthorn, blackthorn, dogrose, European spindle and guelder rose. Alternatively, consider a sturdy supporting structure and climbers like star jasmine, climbing hydrangea and honeysuckle.

Rule #6: Maintain reversibility

The old building might have stood for decades if not centuries, but that doesn’t necessarily mean your extension will. When the site passes out of your hands and into the care of someone else, they might choose to remove your work entirely, knocking it down and taking the whole site back to the brickwork. With that in mind, it is important to do two things: firstly, create something that is so stylish, sleek and in-keeping that the next custodian will not want to knock anything down in the first place. Secondly, make sure that the process of extension removal is an easy one – doing so must not damage the existing structure in any way. If you have had to apply for Listed Building Consent, it is likely that you will have already had to think about reversibility. 

Nobody likes to think that their hardwork will one day be undone, but it is possible. If a demolition has to take place, at least make sure that it does not damage the original structure in the process.

There you have it – our six rules for adding modern glass extensions to old buildings. Some of these are hard and fast regulations that you need to comply with – others, you can take or leave depending on the way that you want to approach this project. Whatever you do, our seventh unofficial rule is to consult with dedicated specialists who can provide honest, impartial advice, like ourselves.