Prince’s Gate Apartment

Double Storey Extensions: Costing, Planning and Designing

Want more living space but not sure whether to move or improve? Making a two storey addition will improve your home’s layout and increase its value. With the exorbitant stamp duty payable on London homes, shrewd Londoners have been quick to recognise that extending the home you’re already in gives you more bang for your buck. Having made the decision to extend, it quickly becomes clear that including a second storey extension gives you double the space of a one storey extension at a much lower cost per square metre.

With the high cost of housing in London, this is an especially cost-effective option because it will dramatically increase the value of your property as well as providing that much needed extra space. At the same time, it gives you the chance to transform the look and feel of your home both upstairs and down and create the home you truly want, without the high costs and hassle of moving house.

Our expert guide for 2020 will help you cost, plan and design your project perfectly. We’ll walk you through planning permission, building regulations and types of double storey extension, and give you some design pointers along the way.

Double Storey Extension or Single Storey – Which is Best?

The advantages of a double storey extension are twofold – firstly, you can add additional bedrooms as well as extra living space. Secondly, a double storey extension is more cost effective than a single storey extension of the same size. You may wonder why, and it’s easy to explain: how ever many storeys you go for, your extension will always need foundations and it will always need a roof. This means that the cost of each additional storey is simply for construction of the floor and walls. The cost of a double storey extension is therefore significantly lower per square metre compared to a single storey extension providing the same square footage of additional space.

How Much Does a Double Storey Extension Cost?

The burning question is how much does a two storey extension cost? That will very much depend on what you want in terms of space, layout, fixtures and finishes. It will also depend on the complexity of the design and the nature of the site. For example, an extension on marshy or sloping ground will cost more than one built on flat solid ground because of the additional engineering involved. Below is a guide to double extension costs in London:

What is the Average Cost of a 2 Storey Extension?

As a rule of thumb you can add about 75% of the cost of a single storey extension for each additional storey. The average cost per square metre will vary according to the quality of build, fixtures and finishes you want. Below is a comparison table which gives you a rough idea of how the specification (the type and quality of the materials, roofing, windows and doors) affects the average cost of a 2 storey extension. Please note these figures are for build only and do not include the cost of finishes.

Type of Extension Average Cost of a 1 or 2 Storey Extension in London per m2 (before finishes)
Basic Spec Medium Spec High Spec
1 Storey Extension £1,500 £1,950 £2,500
2 Storey Extension £1,250 £1,700 £2,250
Refurbishment of Adjoining Areas £1000 £1,500 £2,100

In addition to the construction costs you will need to pay design fees, engineer’s fees, planning application, building regulations application and party wall survey (if applicable) which will all typically add about £1900-3200 to the cost of the build. All works will be subject to VAT at 20%.

Example Double Storey Extension Cost in London

Specification for typical 4m x 5m (20 m2) footprint, giving 40m2 additional space over two storeys Example double extension cost in London (build only) Typical cost of fees & VAT (excludes finishes)
Basic Spec £50,000  



Medium Spec £68,000
High Spec £90,000
Refurbishment of Adjoining Areas £30,000—£90,000 depending on the standard of specification. This often includes installation of a new kitchen.

*Depending on who you hire. For example, an award-winning, high-end architectural studio will charge a lot more than a freelancer or a design and build company. Similarly, with surveys, a large company will charge more than an individual. See breakdown below.

Building Site Constraints

If you have a complex site, you may have to allow for additional costs for your two-storey extension. Issues may include:

• if you have a soil type that needs a particular building technique or material. For example, clay, sand, marsh or made-up ground

• any structural changes to existing walls may involve additional costs

• if gas meters, pipework, drains, etc, have to be moved

What Fees Will I Have to Pay?

In addition to the construction cost of your double storey extension in London, there may be certain fees to factor in. These could include:

Design Fees:

The role of an architect is to take a client’s broad idea and turn it into reality. They focus on the aesthetic side of the build but also ensure that the finished project will be practical and will gain planning permission and building regulations approval. The design costs can vary a lot. Options are:

 Hiring an architect – Architect’s fees are usually 7% to 15% of the construction value in London, depending who you hire.

‣ A double storey extension will need building regulation drawings. You can expect the design fees for these to be between £3,000 and £5,500.

‣ Planning drawings costs will come to approximately the same.

• Hiring a design consultant – can be a cheaper option than hiring an architect.

• Hiring a design and build company– a cost-effective option because their quote for your double storey extension will include all the design fees. Their quote may include planning fees too.

Structural Engineer’s Fees:

A structural engineer calculates the weight of a planned structure and specifies things like the correct steel beams and other supporting elements of the build. Usually this includes the foundations – how wide and deep they need to be, and whether they need reinforcements or piling.

For most small projects, a structural engineer’s job is straightforward and doesn’t cost much. In practice, your architect will often do the work for you. For more complex projects, like multi-storey extensions, basement extensions or builds where supporting structures are being moved, a structural engineer is essential.

A structural engineer’s fees will depend on the size and complexity of the project. For a simple, single-storey extension being built on flat ground, a structural engineer’s fees might be as little as £400. For a more complex build, like a two-story extension which is built on sloping ground, a structural engineer’s fees can cost in excess of £2,000. Typically, structural engineer’s fees in London fall in the £900 to £1,800 range.

Planning Fees:


Double storey extensions in London sometimes come under permitted development (not requiring planning permission). If you don’t need planning permission, it’s still a good idea to get a Certificate of Lawfulness from your council (approximately £100) to prove your project has been built in compliance with building regulations.


If your planned double storey extension does require planning permission, application costs vary from council to council within London but are about £210. If your planning permission comes with planning conditions, you will have to pay £34 for their discharge before building can begin (a process which can take up to 8 weeks).

Party Wall Agreement Fees:

If your proposed double storey extension requires building or excavating under or near a shared party wall, you will need a Party Wall Agreement. The cost will depend on whether you or your neighbours want or need to appoint a party wall surveyor.

On average, you are looking at a Party Wall Agreement cost of £700 to £1,500 per neighbour. If your neighbours agree to the proposed work and none of you want to hire a surveyor, you can serve the notices yourself and you won’t need to pay surveyor’s fees.

Consents for Alterations to Listed Buildings and Buildings in Conservation Areas:

If you live in a listed building and want to make alterations to it, you must apply to your planning authority for listed building consent. You can usually apply online. Check your planning authority’s website for details. There’s no listed building consent application fee but there is usually a charge to apply for planning permission. Planning authorities will usually process applications received for listed building consent and planning permission for the same property at the same time.

Conservation area consent controls alterations to unlisted buildings in conservation areas. The consent process is similar to the listed building consent process. Your planning authority can advise you.

Building Control Fees:

Once your double storey extension is complete, you will need a building control certificate to certify that all the work meets building regulations. The cost varies depending on the size and nature of the works. An average double storey extension in London will cost around £750. You can hire a government-approved inspection company or liaise directly with your local authority.

Other Fees:

Some double storey extension projects will need other surveys to be carried out. For example, you may need:

• a survey of the existing building to assess structural condition and feasibility £500—£1,500

• a tree report – if you need to remove, replace or move any notable trees £250+

• a flood risk assessment – if your area is liable to flooding £250+

• an ecology report – if it’s a requirement of your planning permission, it may be necessary to assess the impact of    your project on local wildlife £400+

• an archeological report – if your house is inside or close to an archeological area (although this is not normally required for small builds)

• a historic building report –­ if your house is a listed building or is located inside a conservation area

• The cost of these reports will vary depending on who you hire to do them.

Cost of Fitting Out Your Two Storey Addition

The cost of fitting out a two storey extension will be determined by the specification of fixtures and fittings and what you’re adding – a kitchen downstairs will be more expensive than a living room; a bathroom upstairs will cost more than a bedroom or home office. Some general guidelines as to what to expect are outlined below:


• Plastering or dry lining –

‣  If an existing wall only requires skimming, it should cost around £20 — £22 per square metre

‣  Drylining a wall will cost around £25 per square metre, plus a skim coat of plaster at around £22 per square metre.

‣  Doing the same for a ceiling will cost around £50 per square metre.

‣  Wet rendering or plastering will cost about £30 per square metre

‣  Lime or clay plaster in some older or listed properties will cost a bit more

‣  Plastering a fireplace will cost in the region of £15 per square metre

‣  Each plastering job can present its own challenges in terms of the amount of preparation work needed, or how easy     it is to gain access. For example, higher points may need scaffolding or similar access equipment.

• Painting – will cost £60—£80 per square metre depending on the quality of the paint

• Flooring – for a supply and fit, you can budget in the region of £50 to £150 per square metre depending on the specification.


For a kitchen  a reasonable budget would be £8,000—£25,000 depending on the specification. If you’re really going high-end, you can expect to pay as much as £75,000 at the most expensive end of the high-end kitchen range.

Bi-fold or sliding doors – will cost you £1,500—£3,000 per metre.


A new bathroom – you can expect to pay £4,500—£11,000 and upwards, depending on the specification

A shower room – expect to pay £4,500 to £11,000, depending on the specification.


The cost of adding heating will vary. Options are outlined below.

Extending your existing central heating system – will probably only be a few days’ work for a plumber. You can reckon on it costing £1000 upwards.

Underfloor heating – is more expensive, but is worth considering for large, open-plan spaces like kitchen diners or conservatories. You can expect to pay in the region of £70—£130 per square metre, depending on the system you choose to install. Electric underfloor heating is cheaper than water-fed.

New boiler – you may need a new boiler if you are significantly extending your existing heating system. You can expect to pay around £2,500—£6,000, depending on the brand you choose, and whether you need a combi boiler or a Megaflo system.

Will a Double Storey Extension Add Value to My Home?

In London, a double storey extension can certainly add value to your home. It’s all about supply and demand. But even a simple renovation can increase your asking price. How much a double storey extension in London adds to the value of your property depends on retail value per square foot in your area. But it’s almost always a cheaper option than moving because you’re channeling the money that would go on legal fees and stamp duty into increasing the size and value of your home instead. In London, a typical double extension cost is £1500-£2500 per square metre, with the value of additional space at £4000+ per square metre even in the cheapest boroughs. In some areas of London that additional extra space could be incredibly lucrative.

Building Regulations and Planning Permission for a Double Storey Extension in London

Do I Need Planning Permission for a Double Storey Extension?

Permitted development (not requiring planning permission) conditions are stricter for double storey extensions than for single storey extensions. While it’s sometimes possible to build a double storey extension under permitted development rights, it’s more likely that you will need planning permission. You will need planning permission for your proposed extension if:

• It covers over half the area of land around your house

• It extends towards a road

• It results in an increase in the overall height of the existing building

• It extends more than 6m from the rear of a semi-detached or terraced house

• It extends more than 8m from the rear of a detached house

• It is taller than 4m

• It’s width is more than half the width of your house

• It uses different materials to the original house

• It includes a balcony, terrace or raised veranda

Permitted Development:

To build a double house extension under permitted development, you will need to build within the constraints outlined below. Rules for permitted development are stricter in conservation areas.


Area – the total area of your extension must be no more than 50% of the total free area around the house

Width  the width of your double storey extension must be less than 50% of the width of the existing house

Length – the maximum length for a rear extension on a terraced or semi-detached house is 3m, and 4m for a detached house.

Height – the height of a two storey extensions must not exceed the height of the eaves on the existing house.

Distance from the Boundary:

If the distance to the boundary is less than 2m, you can only build a single-storey extension under permitted development.


Your extension must blend in visually with the existing house and the neighbourhood.

Balconies and terraces:

Balconies and terraces are never allowed within permitted development.

Certificate of Lawful Development:

If your project doesn’t need planning permission, it’s still a good idea to get a Certificate of Lawful Development from your council once the work is complete. This certifies that your project has been built within the constraints of permitted development. The fee for the Certificate of Lawful Development is £103.

Applying for Planning Permission for a Double Storey Extension:

If your proposed double storey extension can’t be built under permitted development, you’ll need to file for planning permission. A complex project might also include a pre-planning process. If you hire an architect or a design and build company, they will handle the planning process for you and submit all the necessary documents to the appropriate authorities.

Does My Double Storey Extension Need to Comply with Building Regulations?

All building work needs to comply with building regulations. If you hire a professional building company, they’ll liaise with the local building control officers during your project. They’ll also provide you with a building regulation certificate at completion.

Do I Need a Party Wall Agreement for a Double Storey Extension?

You need a party wall agreement if you are building near or altering a wall that’s shared with a neighbouring property. You may also need a party wall agreement if you are excavating foundations within 6m from the boundary, depending on the depth of the foundations.

How Big can My Double Storey Extension Be?

Planning rules limit how big a double storey extension can be.


A two storey extension must be no higher than the existing roof eaves. If this results in ceilings that are too low, there are possible solutions like leaving ceiling joists exposed on the upper floor to make it feel more spacious. You can also make a shallow excavation at ground floor level to achieve the additional ceiling height.


Planning rules limit the extent to which you can extend. The extension must project no further than a line set at 45 degrees from the centre of neighbours’ windows. This is to prevent loss of light to their property.

How Long Does It Take to Build a Double Storey Extension?

The time it takes to complete a two storey extension varies a lot. Much of this is down to how long planning permission and things like party wall agreements take which can be difficult to estimate. To give you a rough idea on timing, average times for each stage are given below. Hiring a design and build company can often get your project completed faster because they are able to overlap the design and build phases.

Planning Time:

• Design – 8 weeks

• Planning – 8 to 12 weeks

Preparation Time:

• Party wall agreements – 2 weeks to several months, depending on how straightforward negotiations are

• Technical drawings, building regs, structural design – 4 to 8 weeks

Construction Time:

Construction of a double storey extension takes about 14 to 16 weeks. Rough design and build timings are outlined below:

• Excavation and foundations – 1 to 3 weeks

• Structural framework – 1 week

• External walls – 2 weeks

• Ground floor slab and second storey floor – 1 week

• Roof – 2 weeks

• Dry lining and plastering – 2 weeks

• Electrics and plumbing – 1 week

• Windows, doors, flooring, skirting and other joinery – 2 weeks

• Painting and decorating – 1 week

Types of Double Storey Extension

There are various options for your double storey extension:

Double Storey Rear Extension

A two storey rear extension is a popular choice for detached and semi-detached houses and is sometimes feasible for terraced houses too. The extension is added to the back of the house and extends into the back garden.

Double Storey Side Return Extension

A two storey side return extension is built in the narrow space along the side of the house. It’s a great option if you don’t want to reduce the area of your back garden. However, if neighboring properties have overlooking windows, you may be limited in what you can do with this type of extension.

Double Storey Wrap Around Extension

A two storey wrap around extension is one of the best solutions for maximizing space. It extends the property backwards and sideways.

Double Storey Front Extension

A two storey front extension is relatively uncommon but is a feasible option, especially if you have a large front garden. Because it has the greatest impact on the appearance of your house, there are stricter rules governing what alterations can be made.

What is the Best Direction for Me to Extend In?

If your garden is big enough, the easiest option is usually extending backwards. If there are no overlooking windows from your neighbor’s property, a side extension can be great because you don’t lose any of your back garden. It’s worth checking whether your neighbour wants to do a side extension as well – you have the option of filing a joint planning application to use all the space between your properties. An architect will help you understand all the options open to you.

Designing a Double Storey Extension

Designing your two storey extension is the point at which you can make decisions on exactly what you want in terms of function, layout, features and finishes so that you get the home you truly want, tailored to your exact needs.

Interior Layout of Your Double Storey Extension

The layout of your extension will be determined by what you want in terms of function, space and light. It is worth consulting a professional designer to help you get the most from your project.


Building a double storey extension is an opportunity to improve the layout of the existing spaces downstairs. You can create an open-plan living room or a large kitchen-dining space for entertaining family and friends. It is also an opportunity to make designated space for utility and get rid of wasted space in hallways and corridors.


A double storey extension gives you the chance to remodel the upstairs too. Extra bedrooms and en-suites always add value to your property.

Exterior Materials for Your Double Storey Extension

Take the chance to increase the kerb appeal of your property. The easiest option for planning permission is to match existing materials but it’s also an opportunity to create something truly stunning that blends contemporary and period architecture. An architect or designer will be able to go over the options with you so that you can make the best choice for you.

Doors and Windows for Your Double Storey Extension

A double storey extension often results in the loss of the existing openings. This provides you with a fantastic opportunity to place new doors and windows for optimum function and light. You can install bi-folding or sliding doors, roof lights, glazed walls and structural glass to maximise light or you can choose to match doors and windows to existing ones.

The Roof of Your Double Storey Extension

The roof design of your two storey extension will be subject to planning requirements. But there are a range of options open to you from green to flat rooves, to something that matches the style of the existing roof.

Bringing the Outside In

Building a double storey extension is an excellent opportunity to open up the house to the garden and bring the outside in. Bi-folding or sliding doors, glazed walls and structural glass all maximise light and allow you to enjoy your garden year-round.

Need Help Planning a Double Storey Extension?

Thinking of building a double storey extension but not sure where to start? With over 15 years’ experience in delivering high-end double storey extensions in London, we can help you realise your vision – engineered to perfection and delivered on-time, on-budget and defect-free. You are assured of:

• accurate, fixed-price quote and detailed cost breakdown at the outset – no spiraling costs and no surprises!

• structural expertise at all stages of design and build

• high quality design, construction and finishes from skilled designers, construction experts and specialist craftsmen

• full transparency – follow the on-site progress of your project online through videos and photos as they are updated each week; view milestones and percentage of work complete, and monitor cashflow.

Contact Dobuild expert house renovation team today to learn how we could benefit your double storey extension project. Call us on 0203 916 5327 or email us at to arrange a free no obligation consultation and quote.

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