Planning and costing a two storey extension

A cost-effective two storey extension could make more sense than moving if you need more space. Expert renovator Michael Holmes shares tips for getting yours right…

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If you're thinking of extending your home, a two storey extension is a cost effective option that will transform the look and feel of your home. Perfect for small homes and growing families, it's the easiest way to increase living space and add an extra bedroom.

It may also prove more cost-effective than moving house or building a single storey extension or outbuilding. A two storey extension spreads the cost of the foundations and the roof – bringing down the average construction cost per metre squared.

Are two storeys really better than one?

‘An extra storey doubles the amount of additional living space but, more importantly, it also offers great value for money,’ says Melanie Clear, director of Clear Architects. ‘A roof is needed whether you choose a single- or double-height design, so you are really only paying for additional floor joists and extended walls to achieve much more space.’

Whether an extra storey will increase your home’s value, however, is not as straightforward. ‘The property market is incredibly diverse at the moment. Prices can be going up, down or staying the same even within neighbouring streets,’ says Kate Faulkner, managing director of ‘If the market is rising, the extension is more likely to add value than if it is falling.’

Value is also influenced by supply and demand, so you should consider your local market, too. ‘In some areas, such as London, the addition of extra space is tremendously rewarding financially, so to buy a home with an extra room here might cost you £100,000, whereas in cheaper parts of the country you might actually find it more cost-effective to buy a bigger home,’ explains Kate.

an extended and remodelled ground floor falt has been transformed with a sympathetic but modern two storey extension

Coupdeville Architects helped the owners of this flat in a London terrace reimagine the space with a two-storey extension and remodel

(Image: © Coupdeville Architects)

How much will a two-storey extension cost?

Labour costs vary across the UK – and the design, build spec and materials you choose will affect the final price. As a guide, the finished cost per square metre for a standard-quality two-storey extension will be £1,320 to £1,620; for a good quality design it’s £1,620 to £1,860; and for excellent quality, £1,860 to £2,100. The cost of fitting out your two storey extension will depend on the rooms within it.

  • For a kitchen, budget from around £5,000 to £20,000, depending on the specification; if you go high end, costs can increase significantly on this.
  • For a bathroom, factor in from around £4,500 to £11,000, according to the level of fittings.
  • For a shower room, plan for between £4,500 and £11,000, again depending on your level of fittings.
  • For flooring, budget in the region of £25 to £100 per square metre.
  • For wall and ceiling finishes not included in the build costs, set aside from around £85 per square metre for plaster or dry-lining plus paint.

The cost of project management is another big factor. A building contractor or architect will charge 15 to 20 per cent on top of the net cost of labour, materials and overheads. But you can save money if you manage your own project.

Remember to consider VAT, which is at 20 per cent of the labour, materials and services. If your individual sub-contractors turn over less than the VAT threshold, they won’t charge you any VAT on labour. This can result in a big cost savings.

If you have time and good DIY skills, doing some of the work yourself will reduce the cost of your extension. There are big savings to make through DIY, but it can prove a false economy if you take on too much work yourself. ‘Building a two-storey extension is a major project and should only be tackled by the most competent DIYer or builder with a wide range of skills. Even then, you should consider bringing in specialists for electrics and plumbing,’ says Michael Holmes. ‘Technically the only part of the project you cannot carry out on a DIY basis is gas installation, but on such a large-scale extension, it is likely you’ll need help at several different stages.’

Get a comprehensive extension cost calculation using our free extension cost calculator

Two storey extension with white render on the rear of a traditional property in London

This two-storey rear extension creates an open, social space, which includes kitchen, dining, study and relaxing areas at ground level. Upstairs, there is a further modest extension to the family bathroom to allow for a separate bath and shower. The project, by Selencky Parsons architects, cost £200,000, including a loft conversion 

(Image: © Selencky Parsons)

Which type of design will suit my house?

‘The first thought is often to add more space in exactly the same style as the existing property,’ says experienced renovator Michael Holmes. ‘However, this is difficult to get right, as not only do you need to match the materials, but you also must take ageing and architectural details into account. It is often easier to create a complementary design instead, such as a glazed extension to a traditional cottage. Alternatively, opting for a different period style is a technique that can help the new extension to look established quickly.

‘Where the style of the existing property is unappealing, an extension can form part of an overall remodelling scheme, changing the form of the building, the materials and detailing, resulting in a full transformation.’

Getting the interior flow right is also vital. ‘If the extension is modest in scale, use it to enlarge existing rooms instead of creating additional independent spaces,’ adds Michael. ‘Avoid turning rooms into corridors by planning new circulation routes: key rooms, such as the kitchen and living room, should always be accessed from the main hallway/circulation space, and you should group rooms with related functions next to each other – for example, the kitchen and dining area.’

How tall can a two storey extension be?

The main limitation on two-storey extensions is usually the roof height. Planning policy requires an extension to be sympathetic to the existing house, so the height of its ridge and eaves should not be taller than the existing roof.

‘Where a building has low ceilings, it can be difficult to build an extension tall enough to integrate two full (2.4m) storeys. However, there are solutions, such as using lower ceiling heights in the new rooms, especially at first-floor level, partially integrating upstairs rooms into the roof space, or setting the extension down slightly in the ground,' explains experienced renovator Michael Holmes.

‘A shallow pitched roof can cover a large extension without being too tall, but this is not always acceptable, especially on a period building or in a sensitive location such as a Conservation Area. A flat roof can cover an area of any size, but is not usually considered acceptable for a two-storey extension, unless part of an overall contemporary design scheme of significant architectural merit. 

'Other solutions for covering large extensions without too much height include creating a series of small intersecting traditional pitched roofs (at an angle of between 40 and 55 degrees), or an area of flat roofing “hidden” behind a more traditional pitched roof.’

How big can the extension's footprint be?

‘Planning rules limit how far a two-storey extension can project, and how close it can be to the boundary, so as to prevent a loss of light to neighbouring properties. It must project no further back than a line set at 45 degrees horizontally from the centre of neighbouring windows – the so-called “sight lines”.’

 Will I need planning permission?

Many two storey extensions can be built under permitted development rights – our guide explains how – without the need for a planning application as long as you stay within these restrictions.

  • Your extension can’t be taller than the highest part of your existing roof;
  • It must not extend beyond the rear wall by more than three metres;
  • It has to be at least seven metres away from the boundary;
  • It has to be built in materials to match the original structure;
  • Further restrictions apply to homes in Conservation Areas (you will need to contact your local conservation office for advice).

If you’re planning a larger extension or one that doesn’t meet these criteria, you’ll need to apply for planning permission from your local authority. The fee in England is currently £206 but you may need to pay for extra reports, such as a Tree Preservation Survey, on top of this.

Even if you don’t need planning permission, it’s worth paying your local authority £103 for a certificate of lawful development. This can be useful when you come to sell the house as it proves the extension is legal and is covered by permitted development rights.

‘Your house must be on quite a generous plot, though, as the extension will need to be built at least two metres from the boundary at the side and seven metres from the boundary at the rear,’ says Alan Cronshaw of Acronym Architecture & Design. ‘In order to be permitted development, the extension will also have to be in keeping with the existing house, with matching exterior finishes and roof pitches where practicable.

‘In most cases you will need to make a planning application to your local authority for a two-storey extension. This involves drawing up plans and elevations and completing forms. After submitting these, it usually takes at least eight weeks for the planning department to make a decision.’

What do I need to know about building regulations?

You’ll need to get building regulations approval. These are minimum standards set to control health and hygiene, safety, welfare, convenience, energy-efficiency and sustainability of a building project. Each local authority has its own table of charges or you can use a private certified building control firm if you prefer.

The size of your extension will affect the fees for building regulations approval. Plan for from £250 for one to 10 square metres, and £900 for 80 to 100 square metres, whether you go for a full plans application with everything approved before you start, or a building notice application, where you need only give 48 hours’ notice of commencement.

Party wall agreements

If any neighbours who share a boundary wall with your property consent formally to the extension, you won’t need a Party Wall settlement and can save money. If not, having a surveyor arrange party wall agreements for you typically costs from £700 to £1,000 per neighbour.

 How can I find a designer and builder?

Always look for a designer, architect or builder who you feel comfortable talking to and can imagine yourself working with. Get at least three written quotes and then compare your tradespeople's quotes carefully. You should also ask to see examples of the architect/designer or builder’s previous work and speak to past clients.

‘Organisations such as the Federation of Master Builders, Home Improvements Guarantee, Which? Local and Checkatrade offer builders who have been assessed in some way,’ says Kate Faulkner. ‘Make sure they have insurance and a warranty service that survives something happening to them or their business, and check they are happy to sign a contract.’

Most architects, designers and builders will offer to manage the project for you for an additional fee of 15-20 per cent of the project cost. This will involve making sure all materials and trades are on site at the relevant time, and managing the timescale and budget. You may want to carry out this role yourself but it can be very time consuming. You could save some of the 15 to 20 per cent if you do.

You can find architectural technologists on and architects via or RIBA.

Our directory of tradespeople is another option for finding sales people. 

A ground floor flat roof extension and first floor gabled and clad extension on a remodelled home

This 1950s property was remodelled and extended by Clear Architects to add two new first-floor bedrooms and luxury features including a wine cellar and cinema room

(Image: © Clear Architects)

 Where should the extension be located? 

The rear of a property is usually the best place to add a two storey extension to a terraced house or semi-detached property.

Where a property has a large outdoor area, as many houses on a street corner do, there may be the potential to extend over two storeys at the side. In some instances, an extension can wrap around multiple sides of an existing property.

If there are roof height restrictions, you could still extend into the roof space to form a one-and-a-half-storey extension. It may also be possible to dig down into the ground and build two storeys without affecting room height. This could result in a basement level or split-level design, and can work well on a sloping site.

Remember that the position of the extension will affect views, garden access, privacy, and which windows will be obscured. It’s also important to consider where the new space will adjoin the existing room plan. Ideally, any new principal rooms should be accessible from the main hallway and landing area. Often, part of an existing room has to be sacrificed to create the circulation space required.’

The style, proportions and materials used for your extension may be limited by what your local authority will agree to. An experienced architect will be able to advise what you can and can’t do. Victorian and Edwardian houses tend to suit both contrasting modern and complementing traditional extensions; homes built from the 1930s onwards, however, usually suit a more contemporary approach.

 Will a two storey extension add value to my house? 

If space is the reason you want to extend your home, look at local houses with the footprint your home will have after the extension. Adding an extension may cost more than the value it will add to the property in the short-term, but it can still work out much less expensive than moving to a larger property. Remember to factor in estate agent fees, legal fees, Stamp Duty and removals fees when considering whether to move or improve.

Properties are generally valued on the basis of price per square metre. To get an idea of local property values, find several properties that are similar to what you propose to build. Then divide the asking or sale price by the area of the property. This will give you an average value per square metre.

If you compare this with the average cost per square metre of your extension, you can work out whether your proposals will increase the value of your property. Be aware that there is a limit to how much value you can add to your home, known as the ceiling value.


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