Extending Your Home

Double-storey extensions: an expert guide to costing and planning a two-storey extension

Double-storey extensions give you double the space of a one-storey extension but you'll spend less per square metre.

Get yours right, and it might make better financial sense than moving to a bigger house.  Here's how:


                           (Image credit: Joakim Boren/Mulroy Architects)

Double-storey extensions are a truly cost-effective way to extend and a sensible alternative to moving house when you’re ready to size up. Unlike one storey additions, they can not only add valuable living space, but extra sleep space too. Plus, double-storey extensions will remedy interiors that are not fit for contemporary life – a lack of bathrooms to balance bedrooms upstairs and, on the ground floor, a layout that gives you a poor connection to the garden, a cramped kitchen, or closed-off rooms where a more open-plan arrangement would be preferable. 

Creating new rooms upstairs and down that fully integrate into your home is the key to a successful double-storey extension. The principal rooms of your home should be accessible from the main hall or circulation space in an open-plan layout, as well as from the landing. Rooms or zones with related uses – like the kitchen and the dining space – work best grouped next to each other. 

Finally, a two storey extension is a cost effective option, giving you more square footage for your buck than a single-storey extension, and allowing you to transform the look and feel of both upstairs and down. Perfect for small homes and growing families, it's the easiest way to increase living space and add an extra bedroom.

Will a double storey extension add value to your home?

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 house.

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.

                   (Image credit: Chris Snook)

Is a double storey extension really better than a single storey?

‘An extra storey doubles the amount of additional living space but, more importantly, it also offers great value for money compared to building a single storey extension,’ 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 Propertychecklists.co.uk. ‘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.

                 (Image credit: Coupdeville Architects) Coupdeville Architects helped the owners of this flat in a London terrace reimagine the space

                 with a two-storey extension and remodel

You could even maximise the space benefits by considering three storeys – achieving permission is more likely if other houses in the street have similar additions. 

This town house (below) was expanded with a tiered ‘wedding cake’ of extensions, providing additional space for playing, sleeping and bathing, while maintaining the roof terrace that had first attracted the owners to the house. Clever glazing solutions make the most of the views and a small extension at roof level draws light down through the existing stairwell.

                (Image credit: Joakim Boren/Mulroy Architects)


How tall can a double 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 a double storey 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”.’

                                   (Image credit: Grant Architecture and Interiors)

How much would a double storey extension cost?

Labour and build costs vary across the UK – and the design, build spec and materials you choose will affect the final price. As a guide, the finished build cost per square metre for a standard-quality double 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. 

So, for a standard 4m x 5m footprint double storey extension (so a gained space of 40m square over both floors), you could expect to pay upwards of £53,000 for build costs alone. This is without professional fees and VAT or interior fit out costs (more on this below), however, and in some parts of the country, such as London, costs will start much higher.

To be on the safe side, it's worth budgeting at the very least £70,000 to £80,000 for a two storey extension with a standard finish and a footprint of 4m x 5m. This excludes interior fit-out costs (more on those below).

For an accurate estimate of how much your single storey extension will cost, use our extension cost calculator.

Professional, planning and other fees for two storey extensions

Other associated costs for a double storey extension could add up to another £25,000, if you were to include the majority of these fees (not all of which will be necessary, depending on what you are planning):

  • Architectural fees, which will be around three to seven per cent of the construction cost; planning drawings and construction drawings cost around £2,700 each.

  • Structural engineers' fees cost around £500 to £4,000 (if roof joists and foundations are specified);

  • Surveyors' fees, if a survey of the existing house is required are somewhere between £500 and £1,500;

  • Planning fees for a residential two storey extension are around £210; 

  • A certificate of lawful development (if needed), is approx. £103;

  • A request for discharging planning conditions costs around £34;

  • Building control charges vary according to your extension’s size, but expect to pay between £200 (for an extension of 10m sq) to £900 (for 80 to 100m sq);

  • A party wall agreement (if needed) will cost £700 to £1,000 per neighbour;

  • Additional fees might include a tree report; a flood risk assessment within flood zones (both £250 upwards); an ecology report (from £400); an archaeological report (possibly several thousand pounds); a historic building report, likely if your home is listed;

  • Project management by a building contractor or architectural designer will be 15 to 20 per cent on top of the net cost of labour, materials and overheads. You can save money if you manage your own project;

  • 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

How much does fitting out a two storey extension cost?

The cost of fitting out a two storey extension will largely be dictated by the room types you’re adding – a kitchen downstairs will be more expensive than a living room; adding an extra bathroom upstairs will be more costly than another bedroom or a home office, for example. Add these figures to your budget for a good idea.


  • Plastering or dry-lining and painting will cost around £85 per square metre;

  • For flooring, budget in the region of £25 to £100 per square metre;

  • The cost of adding heating will depend on all sorts of variables. Extending an existing central heating system may only need a few days’ work by a plumber, at around £150 per day (excluding materials).



  • 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;

  • Bi-fold or sliding doors will cost between £1,500 to £2,000 per linear metre;

  • Underfloor heating will be more expensive, but is worth considering for a large, open-plan kitchen diner, orangery or conservatory. Electric underfloor heating is a cheaper installation choice, some elements of which you can do yourself, but will be more expensive to run than water-fed underfloor heating, which has a more expensive installation cost. You may also need a new boiler; expect to pay around £2,500.


  • 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.



How to cut the cost of a two storey extension

If you have time and good DIY skills, doing some of the work yourself will cut extension costs. 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 double 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.’










                    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 cost £200,000, including a loft conversion 

(Image credit: Selencky Parsons)

Planning a double storey extension

Planning a double storey extension in careful detail is essential if you are to finish the build on schedule and on budget – and if it is to meet all your expectations. From checking whether you need planning permission – likely with two storeys – or can extend under permitted development rights – not so likely but possible – to finding the a builder experienced in constructing two storey extensions, good planning will save you time, hassle and money later.

Where should a double storey extension be located?

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

At the side: 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.

An extension to the rear can often be the best place to build onto a semi. Is yours a terraced home or on a  corner plot? Then the side return might be the area to use. Read our guide to adding a side return extension for more tips.

On multiple sides: Super-sizing? Adding two storeys on multiple sides is also possible. Remember, permitted development rights only apply to rear double-storey extensions – so you’ll need to get permission if your plan is to site the extension elsewhere.


Adding a double storey extension to a semi detached house

We're already covering the rules for extending to the rear of a property – semi-detached, terraced or detached in our section on planning permission and permitted development, below. But what if you want to add a two storey extension to the side of a semi-detached house, whether into an area that's currently little used garden space or perhaps an existing garage? 

Here, it is very unlikely that a two storey extension can be built under permitted development rights, especially if the addition is within 2m of the property's boundary. So, whether you are planning to build from scratch at the side of your home or to use the foundations of the existing garage to build a new room above, it is best to seek advice first, then permission from the council's planning department. And, be prepared for the extension to look what's called 'subservient' to the original house – the roofline, for example, will need to be lower than that of the main roof.

As for costs, providing the existing garage foundations are deemed to be able to take the load by an engineer or surveyor, building a new second floor extension on top will be more cost-effective than knocking down an old garage and adding an upper floor or building a new two storey extension from scratch. A typical second floor extension above an attached single garage would cost £900 to £1,500 per m². Building regulations would apply.

Can I add a double storey extension to a flat?

If you own a flat or maisonette that comprises the two lower storeys of a house a two-storey extension is in theory possible, but seek legal advice regarding the freehold and leasehold arrangements with the other flats in the building. Do talk informally to your neighbours first to get them on side. Just as when extending a freehold house, planning restrictions on the height, size and position and party wall agreements will all come into play. In this two-storey extension to a ground floor and basement maisonette within a Victorian terraced house, Gundry & Ducker Architecture replaced ramshackle lean-to structures with an extension, forming a living/kitchen/dining space downstairs plus a studio room upstairs. 

                         (Image credit: Andrew Meredith/Gundry & Ducker Architecture)


What negative implications might the extension's position have?

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.

Think about the possibility of light reduction to neighbours’ windows when building an extension to your home as they could complain that you haven’t considered their ‘right to light’. If you haven’t taken this into account, your neighbour may oppose your building project – even if you’ve been granted planning permission. A court can award compensation, request modification or, in the most extreme scenario, prevent the work from going ahead.


Note that there isn’t a statutory right to light in Scotland.

                   (Image credit: Rick Mccullagh)

How can you work around a double storey extension's roof height restrictions?

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.

Does a double storey extension 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).


Even if you don’t need planning permission, it’s worth obtaining 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.

If you’re planning a larger two storey extension or one that doesn’t meet these criteria, you’ll need to apply for planning permission from your local authority. 

‘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.’


Does a double storey extension need to comply with building regulations?

You’ll need to get building regulations approval for the works carried out.

Each local authority has its own table of charges or you can use private certified building control inspectors.

text credit:  REALHOMES.COM   --------------------------------------------------------------------------