Double Storey Extension [2019 Guide]: Cost, Planning, & Design Tips

A double storey extension is an excellent choice to maximise the space in your property, instead of moving to a new, bigger place. Usually double storey side and rear extensions are technically simple and are feasible in most detached and semi-detached houses.

Double storey extension vs single storey

A double storey extension is an even better investment when compared to a single storey. The excavation and the roofing costs won’t change from single to double storey extension. This makes the cost per square metre of a double storey extension much lower than the cost of a single storey.

Double storey extension roof view of the back

How much does a double storey extension cost?

The cost of a double storey extension can vary depending on:

  • location
  • site access
  • nature of the soil
  • size in square metres
  • type and shape of the roof
  • finishes like windows, rooflights, underfloor heating, flooring etc

Average cost of a 2 storey extension

The average cost of a double storey extension varies with the location across the UK, going from £1,500 to £2,500 per sqm, excluding finishes.

In London the average cost excluding finishes is higher, between £1,800 and £2,800 per square metre for the building cost. But in London the added value of extra space is also higher, with a minimum of £4,000 per sqm of increased value.

What fees can I expect to pay?

Beyond the construction cost, there are fees you need to budget for when planning a double storey extension.

Planning fees

Double storey house extensions can sometimes be done without planning permission.

In case you need planning permission, householder applications are about £210. The cost can be different depending on the council.

If your planning permission comes with planning conditions, you will pay £34 for their discharge.

If you don’t need planning because your 2 storey extension is built under permitted development, you will still want to get a Certificate of Lawfulness from your council (approx fees £100)

Design fees

Architectural fees are usually between 3% and 7% of the construction cost and they can go up to 15% in London. For a double extension, you will need to produce at least building regulation drawings, with design fees between £3,000 and £5,500. Planning drawings costs are approximately in the same range.

On top of this, you will need to budget for structural calculation. The fee for the structural design would go from £900 to £1,800 for a standard structural design.

Keep in mind that the design costs vary a lot, according to the architects you appoint and whether the design is straightforward or not. Hiring a consultant will save you money, as opposed to appointing an architect studio.
Another cost effective option is to hire a design and build company, that will quote your double extension including all the design fees. Some design and build companies might include planning fees in their quotes.

Party wall fees

If your extension includes building or excavating astride or close to a boundary, you will need party wall agreements with the adjacent neighbours. The cost of a party wall agreement depends on whether your neighbours and you will want or need to appoint a party wall surveyor.
As an average, party wall agreements will cost £700 to £1,500 per neighbour.

Keep in mind that you can serve the notices yourself and won’t need to pay for a surveyor if your neighbour agrees on the proposed work and they don’t appoint a surveyor themselves.

Building control fees

You will need a building control certificate at completion, that certifies that all the work is in compliance of building regulations. The average cost for a double storey extension is £750, depending on the size and the nature of the works.

You can liaise directly with the local authority or engage a government-approved inspection company.

Other consultants’ fees

It is possible that you will need to appoint other consultants, according to the circumstances, for example:

  • a survey of the existing building
  • a tree report: if you need to move, remove or replace a tree
  • flood risk assessment: if you are in an area where floods are possible
  • ecology report: if the authorities think your construction work may affect the local wildlife
  • archeological report: if your house is inside or close to an archeological area
  • historic building report: if your house is a listed building or is located inside a conservation area

The cost may vary, depending on the agency or the consultants you will hire.

Double storey extension roof view of the side

Cost of fitting out your double storey extension

Overall, the average building cost of a double storey extension varies across the UK, going from £1,500 to £2,800 per sqm.

The price of the fit out depends heavily on the usage you’re planning for your extension and the quality of the finishes you are choosing.

Make sure you understand the breakdown of the costs, so that you can invest your budget on your priorities.

General

Plastering and painting costs can go between £40 and £60 per square metre, depending on the type of paint.

Supply and fit of the flooring depends on the cost of the flooring itself. As an average, it can go between £40 and £150 per square metre and more, if the flooring is very expensive.

The cost of the heating is very variable, with underfloor heating being the most expensive option, which can go from £60 to £100 per square metre, including the materials.

You might need to change your boiler to accommodate the additional needs for a bigger area. It may cost from £2,000 to £3,500.

Downstairs

Supply and fit of kitchen can virtually go up to any price, depending on the kitchen you choose. The minimum budget should be at least £5,000, with a good size and quality kitchen to be between £15,000 to £25,000, including the plumbing.

Bifold or sliding doors cost depends on the size of the opening and the product. It varies between £1,500 and £3,000 per linear metre.

Upstairs

For bathrooms or showerrooms, you will need to consider at least £3,500. The cost can go up to virtually any price, based on the quality of the finishings like sanitaryware, shower screens and trays, taps, tiles etc.

Will a double storey extension add value to my home?

The added value of a 2 storey extension goes with the average price per square metre of properties in your area. Sometimes there is no actual added value in the short term, after the costs of building. But if you need more space for your family, it can still be cheaper than moving to a new bigger house.

If you are planning to sell and your house is not in excellent condition, a prior renovation can increase your asking price. At this point you can include a double storey extension, as the cumulative price will be lower than doing the renovation and the extension at two different moments.

In general, double storey extensions are the cheapest extensions per square metre. In London, for example, they usually cost between £1,800 and £2,800, while the property price per square metre in the cheapest London borough would be around £4,000 per square metre.

Double storey extension in Croydon elevation

Planning and regulations for a double storey extension

It is possible that a double extension falls under permitted development rights.

Do I need planning permission for a double storey extension?

Although the options for a double storey extension are tighter than a single one, planning permission is not always needed.

Permitted development rights

To build a double house extension you will need to stay within the constraints below:

  1. Area: the area of extensions must be less than 50% of the total free area around the house
  2. Appearance: the exterior appearance must blend in with the existing house and the neighbourhood
  3. Width: the width must be less than 50% of the width of the existing house
  4. Length: the max length of rear extensions is 3m for terraced or semi-detached houses, up to 4m for detached houses
  5. Height: for 2 storey extensions, the height must not exceed the height of the existing house eaves. Only single-storey extensions are allowed under permitted development, if the distance to the boundary is less than 2m,
  6. Balconies and terraces: they are never allowed within permitted development rights

Rules for permitted development are stricter in designated areas.

Certificate of lawful development

Even if you don’t need planning, it’s good to obtain a certificate of lawful development from your council, after the completion of the work. This will certify that you built within the constraints of permitted development.
The council fee for the certificate of lawful development is £103.

Applying for planning permission

If your extensions cannot be built under permitted development rights, you will need to file for a planning permission.
Depending on the complexity of the proposed work, the process might also include pre-planning.

Appointing an architect is one option, which is cheaper with a freelancer. Another way is hiring a design and build company, which is often the most cost effective approach.

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

All building work needs to comply with building regulations. Professional building companies engage with the local building control officers or with a certified private agency during the project. At completion, they’ll provide you with a building regulation certificate.

Do I need a party wall agreement for a 2 storey extension?

You need a party wall agreement if you are building or altering a wall that sits astride a boundary with an adjacent property.
When excavating the foundations for your double storey extension, a party wall agreement is needed when you are building the foundations up to 6m away from the boundary, depending on the depth of the foundations.

How big can my double storey extension be?

Planning rules include limitations on how big a double storey extension can be.

Height

The height of a 2 storey extension must not exceed the existing roof eaves.
If the resulting ceilings are too low, there are possible solutions, like excavating a few centimeters at the ground floor. Leaving exposed joists at the upper floor is a good option to give a feeling of space and air, even when the actual height is lower than ideal.

Footprint

Planning rules limit how far a two storey extension can project. If your neighbours have window openings towards the proposed extension, there are limitations to prevent a loss of light to the neighbours. The extension must project no further back than a line set at 45 degrees horizontally from the centre of neighbouring windows.

How long does a two storey extension take?

The time it takes to complete a two storey extension can vary a lot. External factors are especially difficult to estimate, like planning and party wall agreements.
It is possible, though, to evaluate average times for each preparation phase. The duration of the construction, on the other hand, is easier to estimate.

Planning time

  • Design: 8 weeks
  • Planning: 8 weeks to 3 months for a successful planning process

Preparation time

  • Party wall agreements: from 2 weeks to months, depending on how difficult the negotiation
  • Technical drawings, building regs, structural design: 4 to 8 weeks

Construction time

As an average, the construction of a double storey extension takes 14 to 16 weeks, depending on the size. This might sound a lot, but it is important to give each phase the appropriate duration for the best results:

  • Excavation and foundations: 1 to 3 weeks
  • Structural beams and columns: 1 week approx
  • External walls: 2 weeks approx
  • Ground floor slab and 2nd storey suspended timber floor: 1 week approx
  • Roof: 2 weeks approx
  • Dry lining and plastering: 2 weeks approx
  • Electrics and plumbing: 1 week approx
  • Decoration and painting: 1 week approx
  • Flooring, skirtings, windows, doors and other joinery: 2 weeks approx
Double storey extension in Gants Hill, London

Types of double storey extensions

According to the relative position to the existing building, there are different types of double storey extensions

Two storey rear extension

It’s an extension at the back of a house, often in the garden. It’s a good option for detached and semi-detached houses. A terraced house extension at the rear of a property is also sometimes feasible.

Two storey side return extension

It uses the narrow space at the side of a house, without using up garden space. It’s a great option for a ground floor extension, to implement a change of character on a ground floor without compromising the back garden area. This is an option for a semi-detached or detached house extension. Keep in mind that neighbouring properties with overlooking windows might limit this option

Two storey wrap around extension

It’s the combination of rear house extension and side extension. It can be a very big addition in size to the existing house. It’s feasible for a semi-detached or detached house extension.
This is the optimal solution to maximise the added space to the existing property.

Two storey front extension

This is the least common type of 2 storey extension. Extending at the front has the biggest impact to the appearance of your house. In all councils this means stricter rules to what alterations can be made.
It can still be feasible, especially for detached houses with a large front garden.

What direction should I extend?

There isn’t one right answer to this question, Usually the easiest option is extending to the rear, if the garden is big enough. Side extensions can be a great option, when there are no windows from the neighbours’ side.
If your neighbour wants to do a side extension as well, you might consider the option of filing a planning application together, to use up all the space between the two buildings.
Having a good architect on your side will help you understand all the options and make the best decisions on the direction of your 2 storey extension.

house-extension-gantshill-facade-double-front-door-white-render-black-details-scaled-min

Designing a double storey extension

Doing a 2 storey extension is not only about space. The way you design the space must meet your priorities.

Interior layout of your double storey extension

Depending on whether you are building a kitchen, a bedroom, a shower room, a playroom, the requirements will be different, and small details can make a huge difference as well.
This is why it is always a good idea to hire professional designers and building company for a project that is often one in a lifetime.

Downstairs

Building a double storey extension is an opportunity to improve the layout of the existing spaces. If you are planning a kitchen extension, you might create a open-plan living room or a larger dining space to receive family and friends.
An extension is also a good opportunity to make space for a utility or a laundry room in addition to the enlarged rooms. Avoid wasting space in hallways and corridors.

A smart placement of the columns can be very important can create leaves an uninterrupted open plan space.

Upstairs

What would you like to achieve with your new space upstairs?
If you are building a new bedroom, for example, the height of the ceiling is not as important as in the living room. Make it an ensuite, if possible, as ensuite bedrooms are an added value to a property.

Remember there are building regulations on the size of staircases, landings, corridors etc. Make sure you seek advice from a professional for the layout design, to avoid issues that may become evident towards the end of the project, when they are costly to resolve.

Exterior materials of your double storey extension

Choosing matching materials for the exterior is the easiest option, especially for the planning. When it is not possible, one good option is to design in contrast with the existing building.
All glazed or naked concrete extensions can result in a stunning contrast for period properties.

Having a good architect or designer on your side means you will be able to understand all the options and choose the best for you: the right creative design can completely transform a property.

Doors and windows of your double storey extension

The positioning of doors and windows is very important for a double storey extension, as it often requires the loss of the existing openings.
The windows should either match the existing or be chosen in contrast.
Bi-folding or sliding doors, roof lights, glazed walls and structural glass are all options that a good architect will help you evaluate to get the best for your priorities.

The roof of your double storey extension

The design of the roof of a 2 storey extension will be subject to planning requirements, but there are still options to make the best choices.
If there are properties around you that will overlook the roof of your extension, a green roof could be a great option to get through the planning.
If the roof surface is not visible from the ground floor level, you can choose a flat roof, making sure that the visible parts still blend in with the existing building.

The indoor-outdoor link

Doing a two storey extension is also an opportunity to get more daylight in and to create a link between indoors and outdoors. That means you can live your garden space everyday, even when the temperature or the weather are not inviting to spend time outside.

Bi-folding or sliding doors to the garden are a great way to achieve that. Glazed walls and structural glass are all options that a good architect will help you evaluate to get the best for your priorities.

Get in touch with us for a chat, a free site visit or a free quotation from our specialists in double storey extensions. We can help with rear and side 2 storey extensions for terraced, semi-detached and detached houses.

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