As someone who inherited a bad extension, I asked the pros whether it's ever worth improving one or if it's better to just start over

When done well, an extension project can not only transform the way you live in your home, but also enhance how it looks, as well as adding to its overall value. However, a bad extension is another matter entirely.

Bad extensions range from those that were poorly constructed at the time they were built – such as the single-skin brickwork kitchen lean-to, complete with corrugated plastic roof, affixed to my own house when we moved in – to those that feature a number of design faults, as well as structures that just haven't stood the test of time.

If you are considering building an extension to replace an existing structure that is currently just not doing justice to your home, be sure to check out this expert advice first – it might just be that there are alterations you can make instead that could save you money while still giving you the new space you crave.

What is a bad extension?

If the term 'bad extension' seems a little vague, that's probably because it is – there are all kinds of factors that go into determining the success (or failure) of an extension. Sometimes you will be putting other people's extension mistakes right, while in other cases time will have taken its toll on a structure.

'A bad extension is typically one that shows poor workmanship, inappropriate materials, or a lack of integration with the original house,' explains Costel Radescu, founder and CEO of CR Group. 'Common issues include inadequate insulation, cold bridging, poor ventilation, damp ingress due to faulty detailing, or mismatched floor levels and roof lines.

'Additionally, non-compliance with UK Building Regulations or a lack of sign-off from Building Control is a major red flag,' continues Costel. 'Structurally, shallow foundations, undersized supporting beams, or incorrect load paths can also render an extension problematic and potentially unsafe.'

'We see common patterns,' reveals Ran Ankory, founder and managing director of Scenario Architecture. 'Extensions that steal daylight from the original rooms and leave the rest of the ground floor feeling dark. Extensions that create a bottleneck between the kitchen and garden instead of harmoniously connecting the two. Or, worse, extensions where glazing has been added without a thorough insulation strategy, leaving you with glare, overheating, drafts or condensation.'

'In our experience the worst extensions were built during the 1980s up to 2000 when the building regulations were not so stringent,' picks up Zaeem Chaudhary director and Chartered Architectural Technologist (MCIAT) at AC Design Solutions.

Ran Ankory

Alumnus of the Architectural Association, Ran co-founded Scenario Architecture in 2007 and leads Scenario’s team. He is involved in the design process, planning negotiations, tendering and realisation of each project, constantly researching new products, materials, design tools and production techniques.

Costel Radescu

Costel is a highly experienced construction professional and entrepreneur, with nearly two decades of hands-on and strategic experience in the UK property and building industry. He started as a labourer when he moved to London 19 years ago, and over the years has built CR Group into a fully integrated property and construction company spanning five specialist divisions, from bespoke joinery and interior design to full-scale developments and property investment.

Zaeem Chaudhary

Zaeem Chaudhary, MCIAT, is director of AC Design Solution, a multi-disciplinary practice delivering architecture, structural engineering, and party wall services across London and the UK.

Should you replace a bad extension?

Whether you have just bought a new house featuring an extension that is letting the whole thing down, or have been living with a poorly constructed addition for some time, is replacement something you should be considering, as opposed to repairing or improving what you have? Much of this decision should be based on just how bad this extension is, according to the experts.

'You should consider replacing an extension when its fundamental structure and performance are working against you,' explains Ran Ankory. 'If there are persistent issues such as damp, recurring leaks, movement, poor foundations, very low ceiling heights, or a roof form that cannot be upgraded without major rebuild, an update can become a cycle of expensive sticking plasters.

'We also advise replacement when the extension is fundamentally the wrong shape for the home,' continues Ran. 'If it creates awkward circulation, blocks natural light to the original house, or wastes floor area with narrow corridors and dead corners, you can spend a lot on finishes and still be left with a space that doesn’t work for your daily life.'

'If there are cracks in the walls or the walls aren't straight, or if the ceiling is too low then it would be better to replace the whole thing,' picks up Scott Young, co-founder of K&S Bespoke Builds. 'Similarly, if it's a conservatory extension and it's draughty and too cold or too hot at certain times of the year, then it's better to replace it.'

'Replacement is usually the better option where the extension’s structural integrity is compromised, for example, inadequate foundations, signs of subsidence, persistent cracking, or roof failure,' adds Costel Radescu. 'If the extension was built to outdated standards and cannot reasonably be upgraded to meet current Building Regulations, refurbishment may just not be viable. Replacement may also be appropriate where the layout, ceiling heights, or construction type no longer suit modern living or energy efficiency expectations, or where the design significantly detracts from the overall value of the property,' adds Costel.

'A practical test is this – if you need to redo most of the structure, most of the thermal envelope, and most of the services to make it comfortable, you are often better off starting again and designing something that properly fits the way you live now,' summarises Ran Ankory.

Scott Young

Scott Young is the co-founder of K&S Bespoke Builds, a construction company in Reading, UK, specialising in conservatories and extensions.

Can you improve a bad extension without replacing it?

In some cases, it might just be a case of making a few improvements and upgrades to an extension – in fact, very often, just as with a house renovation, this can completely transform the space.

'There are lots of things you can improve without removing the entire extension,' points out Scott Young. 'You can re-roof, replace windows, add insulation, re-clad or replace the render, for example.

'If the structure is sound but there's a draught coming through the windows, then just replace the windows,' continues Scott. 'Same for the roof – if a leak is coming through the roof but everything else is fine then just re-roof it. A lot of our work is re-roofing old conservatories and replacing windows on extensions with double glazing.'

'If the structure is sound overall, focus improvements on thermal upgrades, such as improving insulation to walls, floors and roofs in line with Part L, addressing cold bridges, and upgrading windows and doors,' adds Costel Radescu. 'Internally, reconfiguring layouts, improving natural light, and upgrading finishes can transform usability. Externally, improved cladding, roofing materials, and better detailing can help the extension blend more sympathetically with the main house. Even landscaping and improved drainage can also resolve practical issues and enhance overall appearance without full replacement.'

Will replacing a bad extension add value to your home?

What many homeowners want to know before removing an old extension and replacing it with new is whether it will add value to their home – and by how much.

'Replacing a poorly designed or defective extension with a well-built, compliant structure can significantly improve both market value and saleability,' says Costel Radescu. 'A new extension built to current standards will enhance thermal efficiency, improve EPC ratings, and create more functional, attractive living space. Buyers are often wary of substandard extensions, so removing a known issue can reduce surveyor concerns and strengthen buyer confidence.'

'In conservation areas, replacing a subpar extension can increase the value of a property by 8–12%,' reveals Zaeem Chaudrey. 'Properties worth over £1 million may see a £30,000–£50,000 drop in sale prices due to a bad extension. Plus, don't forget that wasted heating costs £800–£1,200 a year due to poor thermal efficiency alone so you will make other savings by replacing it.'

Does it cost more to update or replace an extension?

This will very much depend on the level of improvement required as well as the extension ideas you have in mind for the replacement.

'Renovation is typically 40–60% less expensive than rebuilding if the structure is correctly attached to the main house and the foundations are solid,' advises Zaeem Chaudrey. 'However, replacement is frequently the better option for extensions with structural movement, missing foundations, or high levels of asbestos. For clarity on what you are dealing with, get a structural survey, which costs between £500 and £800.'

'Cosmetic upgrades or minor layout alterations can be cost-effective if the extension is fundamentally sound,' further advises Costel Radescu. 'However, where extensive remedial works are required, such as underpinning foundations, replacing roofs, upgrading insulation to meet current standards, or correcting poor drainage, renovation costs can quickly approach or exceed the cost of a new build. In such cases, demolition and rebuilding can offer better long-term value, improved performance, and regulatory compliance.'

'It depends on what you can keep. If the footprint works, the structure is sound, and there is enough height to add insulation and improve glazing without making the space feel pinched, renovating can be a less invasive solution,' concludes Ran Ankory. 'Our approach is to model options and test them against real daily scenarios. How you cook, how you move through the house, where clutter lands, where you want light, privacy and calm. If the existing shell cannot support those scenarios without heavy intervention, starting again may offer better value for money in the long run.'

FAQs

What are the best ways to improve a bad extension?

If you decide to retain your existing extension and upgrade it, what are the improvements that will make the most difference?

'Start by diagnosing what is actually wrong,' advises Ran Ankory. 'Is it a light problem, a layout problem, a thermal problem – or all three? Often you can make a big difference without demolition by targeting the root cause.

'Light improvements are usually the most transformative,' continues Ran. 'Adding rooflights, adjusting glazing positions, improving internal sightlines, and borrowing light into the original house can completely change how the space feels.'

'The most impactful upgrades include adding high-performance double or triple glazing, adding in proper wall and roof insulation, incorporating underfloor heating and improving connections to the main house through wider doorways or a remodel,' says Zaeem Chaudrey. 'If your extension has a flat roof, replacing old coverings with modern GRP or single-ply membranes with insulation will cost you £80-£120 per square metre but extend the life of your extension by 25+ years.'

If you have decided to remove the old extension do take some time to weigh up the pros and cons of an extension vs a conservatory first. While extensions tend to cost more, they are often better suited to family life – conservatories, on the other hand, are a brilliant way to create a new space with a clear connection to the garden.