Expert advice on adding a small extension

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Renovation expert Michael Holmes

A well-designed, small-scale extension can transform the layout of your home and it needn’t cost a fortune. Here’s some advice on planning, designing and budgeting for a small extension from expert property renovator Michael Holmes.

Sometimes you don’t need to add a large space to create a big impact on the layout of your home. A modest but well-designed addition can be transformative, changing the aspect of a room, bringing in valuable extra daylight, linking two existing spaces, or improving circulation and flow.

Most extensions will be on the ground floor, as this is often where space is needed most, and where there are fewer planning restraints. There are some exceptions, however, including additions to the roof, or the cellar, where only a modest extension can transform the viability of an entire loft or cellar conversion.

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What will it cost?

Expect to pay from £900-£3,000 per m² depending on location, specification and scale. Smaller extensions tend to be proportionally more expensive because of fixed costs such as planning and design fees, Building Regulations fees and the fact that there are no economies of scale.

The simpler the design, the lower the cost, so simple additions such as porches, conservatories and dormer windows – many available in kit form as standard designs – will cost £900-£1,200 per m². More complex contemporary extensions featuring bespoke materials and details throughout will usually be more expensive, ranging from £1,800-£3,000 per m².

DIYers can more or less halve the cost of an extension, as labour constitutes roughly 50 per cent of the construction fees. VAT is another major consideration – most extensions are charged at the standard rate of 20 per cent. Those who manage the construction work themselves using subcontractor labourers, who are not VAT registered, will save the VAT on the labour. DIY project managers also cut out the profit margin and overheads of the main contractor, saving 10-20 per cent – this does need a good knowledge of the building process though, plus considerable skill in scheduling and managing labour and materials. Taking on too much – either project management or DIY – can prove a false economy, so you should tackle only what you have the time and necessary skills to complete effectively.


Planning and permitted development rights

Many smaller extensions can be undertaken under permitted development rights (PD) and therefore do not require planning permission. England, Scotland, Northern Ireland and Wales each has its own PD legislation, and the rules vary in each, so make sure you check the details yourself.

PD rights can be removed or restricted in certain areas, such as National Parks or Conservation Areas, and they do not apply to flats, so it is a good idea to contact your local authority to see whether an application is necessary before undertaking any work.

If you want confirmation that proposed plans constitute PD, then you can apply for a Certificate of Lawful Development (the fee is around £150). It is a good idea to have this in place if you plan to sell within four years, as many potential buyers will need proof that changes are authorised.

If you sell after four years, proof of compliance is not usually required, as unauthorised alteration and extension work becomes immune from enforcement action after four years. There is no time limit on enforcement action on listed buildings. Alterations and extensions to listed buildings will always require listed building consent, even when the work is classed as PD.

If planning permission is required, the fee is £150 and it will typically take eight weeks for a decision from the local planning authority following confirmed registration of the application. For more details, visit


Building regulations

Most extensions, however small, will need to comply with the Building Regulations (Building Standards in Scotland), even if they do not require planning permission. Exceptions include some porches and small self-contained conservatories under 30m².


Design ideas

Add a porch: An enclosed porch can add a useful buffer to the elements, especially if your front door opens directly into a living room. Porch kits are available in PVCu and timber from or

Add a box dormer: A box dormer can transform the amount of full-height space available for a loft conversion. It can be especially useful in providing the headroom needed to add stairs into a loft (1.9m minimum at centre) or for a loft bathroom.

Extend a room: Extensions that add only 1-2m width can’t usefully form a living room in their own right, so it is often best to use the space to extend an existing room, linking old and new with as wide an opening as possible. The exception is if the space is used as a cloakroom or utility room.

Add a link: A covered walkway linking a house to an outbuilding, such as a garage conversion, can be very successful. A glazed walkway can be a great solution to planning restrictions – for example, a listed building, where a solid link would be inappropriate.

Add a lightwell: Adding a small glazed extension to the back of a cellar conversion will fill the space with light. The extension needs only to be modest in scale, and works well if it opens onto a sunken courtyard, with stairs up to the garden.

Add a bay window: A projecting bay window can enlarge a room, and does not necessarily require foundations – it can be cantilevered on brackets from the existing structure. Windows on three sides also bring in extra light.

Fill in the side return: Many terraced and semi-detached homes have a small outdoor area behind and at the side, called the side return. Filling the space with a single-storey extension is a great way to widen a kitchen.

Add a conservatory: An addition that can be built (subject to size and design) without planning permission or Building Regulations consent, this is a great way to create an extra sitting room. To use it all year round, it will need heating and blinds.

Build an orangery: An orangery is the perfect balance between an extension and conservatory. With areas of solid walls and solid roof with a glazed roof lantern, it is better insulated than a conservatory and is easier to control the temperature.

Add an oak frame room: There are many modular extension systems that offer a design-and-build solution for a new room. An oak frame is the traditional addition – a single room with a pitched roof featuring a vaulted ceiling with exposed trusses.

All prices and estimates correct at time of publishing