Building an extension on a house – what to do, where to start and planning permission, explained

Getting the space you need could be simpler than you think…
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  • Hello dream home! By building an extension on a house, even by a few metres, you can truly make it your own and get the space you have working harder to support your lifestyle. We expect so much more from our properties now, preferring to cook and eat together, entertain, work and exercise at home.

    Get your property transformation off to a good start – look at our project planning channel

    A well-executed extension can pay for itself by adding value, too. The good news is that, thanks to Permitted Development rights, you can add space without getting caught up in red tape with your Local Planning Authority.

    Building an extension on a house – where to start

    Building-an-extension-on-a-house-kitchen-Chris-Snook

    Image credit: Chris Snook/Future PLC

    The definitive guide to these is the Government’s Permitted Development Rights for Householders: Technical Guidance. A prize to anyone who can stay awake to the end! The detail does vary slightly in Scotland, Wales and Northern Ireland

    However, we appreciate you might want a more top-line guide, and that’s where we come in. These are the answers to your big questions…

    Related: Extension ideas for small houses to maximise limited living space

    What size extension can I build without planning permission?

    This depends on how big your plot is and what’s already on it. You can build an extension under ‘permitted development’ rules as long as it doesn’t exceed 50% of the area of land your house sits on. That 50% excludes the area covered by the house itself, but does include any existing additions and outbuildings.

    There are other restrictions, too, all relative to your existing building…

    1. Single-storey rear extension planning rules

    At the back of your house, you can add a single-storey extension under Permitted Development, as long as it…

    • Doesn’t exceed 4m from the original back wall (3m if it is not detached)
    • Is less than 4m in height. 
    • In Northern Ireland, a single storey rear extension should be within 3.5m of the rear boundary if the house backs onto a road.

    If you’re attaching this to an existing extension, these measurements must incorporate both the old and new additions.

    You can push it out to 6m (not detached) or 8m (detached), without having to get planning permission, via the Neighbour Consultation Scheme. This gives your neighbours 21 days to object, giving valid reasons. After that, your Local Planning Authority has another 21 days to grant approval and you pay a fee for a Lawful Development Certificate – proof that your Permitted Development project is legal.

    Rear glass box extension

    Image credit: Future PLC

    Related: George Clarke’s extension planning advice – including his brilliant tips for sticking to a budget

    2. Two-storey rear extension planning rules

    You can build a two storey rear extension without planning permission, but it…

    • Can’t go more than 3m beyond the rear wall 
    • Must be within 7m from the boundary of your plot that’s directly opposite that wall (10m in Scotland and 10.5m in Wales).
    • Has to be less than 4m high. 

    If you want to add a storey to an existing single-storey extension, this won’t be possible under Permitted Development Rights if the existing addition extends beyond the rear wall by more than 3m.

    3. Side extension planning rules

    You can build a side extension without planning permission, but only if it is…

    • Single storey and less than 4m in height 
    • No more than half the width of the original house (at its widest point).

    If it will be attached to an existing extension, the measurements above apply to both together as one enlargement.

    If you want your side extension to poke out beyond the rear wall (but not be attached to it), it is subject to the same rules as rear extensions in terms of how far it sticks out.

    There are different rules about on side extensions in Wales – for example, two-storey structures are possible under Permitted Development.

    Kitchen extension in side return with roof windows

    Image credit: Future PLC

    Related: Kitchen extension ideas – to maximise the potential of your extended space

    4. Rear-plus-side extension planning rules

    Tread carefully in cases where both the rear and side wall will be pushed out, as the restrictions on both rear and side extensions apply here – and it can get complicated! Your extension must…

    • Extend no more than 6m beyond the rear wall (or 8m for a detached house).
    • Be one storey, not more than 4m high.
    • Have a total width that’s not more than half the width of the house.

    This makes ‘infill’ extensions, where you’re filling a space between the rear and side wall to square off the footprint, doable under permitted development. Two separate extensions are also possible but connected, wraparound designs will exceed the width restriction and require planning permission (unless you’re in Wales, where the ‘half the width’ rule does not exist).

    Building-an-extension-on-a-house-rear-and-side

    Image credit: Future PLC

    Related: Glass extension ideas – create light-filled glazed spaces to enlarge and enhance any home

    5. Height limit planning rules

    The height of the extension must not exceed the height of the highest roof ridge line (or flat roof) of the existing house. 

    The eaves (where the lowest part of the roof meets the exterior wall) must not be higher than the eaves of the existing house.

    6. Boundary restriction planning rules

    If the extension will come within 2m of your boundary, its eaves can’t be higher than 3m. Plus, if you’re attaching an extension to an existing one which exceeds those limitations, Permitted Development doesn’t apply.

    Can I extend the front of my house?

    If the front faces a road, then no, not under Permitted Development rules. You’d have to apply for planning permission. In cases where your house sits on a corner plot where the side wall faces the road, this cannot be extended under permitted development either.

    If you’re desperate for any extra space at the front you can possibly gain, you could add a porch to an external door (max 3sq m) as long as it’s no higher than 3m and further than 2m away from the boundary with the road.

    If you can’t extend at the front, a porch could still give you useful storage space for coats and shoes

    Related: Extension ideas for bungalows – from dormer loft conversions to elegant conservatories

    What is the ’45-degree’ rule?

    This relates to the definition whether your house fronts onto a road or not, in cases where it’s not clear. If the angle between the elevation of the house and the road is more than 45 degrees, it will not normally be considered as fronting the highway. The same is true if there’s a substantial distance between the house and the road, or if there’s a tract of land owned by someone else between your boundary and the road.

    Do all properties have Permitted Development Rights?

    No. Before you get over-excited and book a builder, the Permitted Development route is a non-starter if…

    • You live in a flat or maisonette. Permitted Development only covers houses. 
    • Your property is listed, or is located in a National Park, the Broads, an Area of Natural Beauty or a World Heritage Site.
    • Your LPA has issued an Article 4 direction. This withdraws PD rights where the character of an area would be threatened.
    • Your property has been granted ‘change of use’ to a dwelling.

    There are also special rules for land classified as being of special scientific interest.

    I need planning permission, so where do I start?

    Most applications take up to eight weeks, unless they are unusually large or complex, in which case the time limit is extended to 13 weeks. If permission is refused, you can appeal but this process can take several months, so you’re better off tweaking your plans to reach an agreement.

    If your project – or your property – is anything out of the ordinary, it’s worth hiring a planning consultant to navigate the permission minefield for you and expedite reaching an agreement to keep your project on track. Just make sure the consultant you engage is a chartered member of the Royal Town Planning Institute.

    If you carry on without planning permission, you could be served with an enforcement notice that orders you to undo all the work you have done. It’s illegal to ignore this, but you can appeal against it.

    Building-an-extension-on-a-house-courtyard

    Image credit: Future PLC

    Do I need my neighbours’ permission for an extension?

    Not exactly. If you require planning permission, the local authority will consult your neighbours as part of the process. If your neighbours object, the authority will determine the impact on them of your proposals and decide if it is acceptable or not.

    It’s best to have broken the news to your neighbours before this stage. You can save time and hassle by ascertaining what their views are and possibly tweaking your plans before you even submit them for planning permission to sidestep objections at the consultation stage. 

    Also, it’s possible that your builders might need to go onto their property during the project and you’ll need your neighbours’ consent for that, so keep them on side.

    There’s also the Party Wall Act to consider – even if your project falls under Permitted Development. If your plans will involve building against or excavating within 3-6m of a party wall you share with a neighbour, you might have to give them two months notice. They have 14 days to either give or refuse consent. A party wall is one that forms the structure of both your properties, but could also be a garden wall that divides your plots.

    Can my neighbour stop or object to my extension being built under Permitted Development Rights?

    Building-an-extension-on-a-house-full-length-glass

    image credit: Future PLC

    No, but it’s in your interests to maintain a good relationship with them, so it’s worth taking their views into account when planning your new extension.

    If you’re going down the Neighbour Consultation Scheme route for a larger extension under Permitted Development, your neighbours do have the opportunity to present valid objections.

    Related: See how an extension transformed this family home in Bristol

    Can I build an extension myself?

    Yes. If you have the time, muscle and a talent for DIY, it’s a way to cut costs and could be straightforward enough for an amateur if it’s a simple design. 

    Chris Thompson, who built his own two-storey extension, says, ‘Being honest about one’s technical and endurance abilities is important.’ 

    By doing it himself, his project cost 20% of the builder’s quote he got at the start. However, he warns, ‘Don’t do it if your sole motivation is money. It’s a big project and if you lose enthusiasm it will come unstuck. You’ll be yet another DIYer with a half-finished project. A desire to accept a challenge and an excitement to do it is what keeps you going when it’s cold and things are taking longer than you hoped.’

    You will be responsible for making sure that the work complies with building regulations (even if your project is covered by Permitted Development Rights). You will also need to do your research on the best materials for the job, which is more technical than you might think.

    For example, roof tiles differ to suit different roof pitches. Contacting manufacturers and reading the literature they send you will take time in addition to the hours you spend labouring.

    You could always take a pick-and-mix approach by doing the unskilled jobs yourself, like clearing the site and digging down for the footings, before handing over to the professionals to tackle the drainage and build.

    Chris took a flexible approach to tackling his build: ‘I did wonder if my brickwork would be up to scratch. I rationalised that I should give it a go and if it looked rubbish the next day it would easily come down again before the cement hardened. If I really had to, I could pay a brickie to do that part instead.

    If you decide to leave it to the pros, the Federation of Master Builders is the best place to start in finding a builder. They’re professionally vetted and come with the benefit of a free dispute resolution service, should things fall apart.

    A partially built rear extension project at a house in Cheshire UK

    Image credit: Alamy

    Do I have to hire an architect?

    Chris, who designed and built his own two-storey extension, says, ‘It is de rigueur, but there’s no reason to use one if you can do the plans yourself.’

    To start with, you have to know what you want and it has to be simple in its style and planning requirements. You should be able to see plans from recent local planning applications on your council’s website, which will give you ideas and potentially a template to follow. 

    You will also need to create highly detailed technical drawings for your builder to follow and submit for building regulations approval. This involves mathematical calculations showing the structural integrity of your proposed extension. As well as considerations around fire safety, energy efficiency, damp proofing and ventilation. 

    ‘These drawings are more demanding,’ says Chris, who did this himself. ‘But all the building regulations are available online. It’s not rocket science, but does take a certain amount of application and understanding.’

    If you have the time, you could also cover an architect’s project-management duties yourself, too.This would mean liaising with the LPA, hiring builders, supplying materials and problem-solving during the work.

    The middle ground would be to DIY the plans but get them checked by a structural engineer. You can use an online plan drawing service, or to use a building firm that offers a ‘design and build’ service.

    Approach the ‘design’ part of this in the same way that you would when hiring an architect and check that it is covered by professional indemnity insurance. This protects you in the case of errors. Ask to see examples of work and get references from previous clients.

    Uncomplicated extensions like this small side addition could be an enjoyable challenge for a confident DIY builder

    Related: Before and After: smart extension turns a narrow kitchen into a cool contemporary family space

    What are the stages of building an extension?

    1. Prep the area

    Start by sorting out adequate access to the site and protecting spaces that are going to be lived in during the project and setting up alternative kitchen/bathroom arrangements if necessary.

    2. Start the groundworks

    Image credit: Alamy Stock Photo

    Next, demolish any unwanted structures and clear the site to be built on. Assign and clear spaces that are to be used for the storage of materials or topsoil. Install gullies and/or soakaway to manage surface water.

    3. Lay the foundations

    Image credit: Alamy Stock Photo

    Dig out and marking out the footings. Lay and connect drains. Put in the damp proof course. Lay hardcore, sand, a damp proof membrane, insulation and concrete to form the base floor.

    4. Start the build

    Image credit: Alamy Stock Photo

    At this point, the walls can go up (usually breeze blocks with a brick outer skin, unless you’re opting for cladding or something more unusual). Window sills and lintels can be inserted as the walls rise. Upper floor joists should be fitted if there is to be more than one storey. 

    5. Add the roof

    Image credit: Alamy Stock Photo

    First, out in trusses and gable walls to form the frame. Add vents, felting and battens, then tile the whole thing. Add guttering, downpipes and flashing. 

    6. Finish the shell

    Image credit: Alamy Stock Photo

    Now you can fit window and door units, and reconfigure your existing interior space by breaking it through to the new part of the house.

    7. Fit the interior

    Image credit: Alamy Stock Photo

    In this last stage, you would lay floor insulation – along with any underfloor heating pipework, if it’s required. Pour a concrete screed to form a subfloor. Complete your electrics, plumbing and plastering. Finally, lay your flooring, fit cabinetry, carpentry and appliances, and decorate the space.

    We hope our guide has given you the confidence to begin your project. Best of luck and we can’t wait to see your transformed space.

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