Front of house

An Expert Guide: Designing, Costing and Planning your Double Storey Extension

When it comes to the square footage of your house, size matters. It affects the property’s market value and the comfort of those who live there. A growing family often require more bedrooms, bathrooms and living space. The recent rise in popularity of hybrid and remote working calls for a dedicated office or workspace at home. This leaves homeowners with a tricky decision. Move to a bigger home, or extend the property you are in?

Moving home has its drawbacks. Leaving a property that you have an emotional attachment to, a lengthy search, bidding wars, moving chains, packing, unpacking, the list goes on. Not to mention stamp duty on London homes is a serious stack of cash. Savvy Londoners recognise that channelling money into improving and extending rather than moving is the most logical answer.

open plan living room with bi fold doors

Once you have decided that extending is the way to go, there is a whole host of options and considerations. The most bang for your buck will be adding a double storey extension to your property. The transformation will be dramatic. You can achieve an expansive interior and an impressive exterior, without the hassle, worry or expense of moving house. There is also the benefit of increasing the value of your property. If you plan it right, you could get a gratifying return on investment.

This expert guide is here for you. It will explain all the ins and outs so you can plan with confidence and successfully turn your property from feeling squashed to spacious. You will get the lowdown on the distinct types of double storey extensions, costing, building regulations, planning permission and a few design tips along the way.

Single Storey Extension vs Double Storey Extension

One of the costliest components of an extension is the foundations. That is why adding two floors is cheaper per square metre than a single storey extension covering the same area. Making it a shrewder investment for your property. Not only is it more cost-effective, but it opens possibilities to expand both the downstairs areas and the upstairs areas. Added bedrooms and bathrooms will improve the comfort of your living situation and increase the value of your property. You can also drastically alter the exterior aesthetic of your property. If you have the budget and ability to go for two storeys when extending, it is the smartest option.


Tackling a project like this will be easier if you have a clear idea of what you want to achieve with the finished outcome. What are your priorities? Is it just more space you are after? More curb-appeal? An area to entertain? A stronger connection between your indoor and outdoor spaces? What do you want the layout and finishes to be? The answers to these questions matter and you should keep them in mind throughout the design process.

To get the best result from your project, it is worth consulting a professional designer. They can give insights on the best layout for your needs, how it will work for you and how to maximise space and light. They may even be able to suggest some budget-saving design solutions.

Downstairs Interior Layout:

Extending your property is an opportunity to reimagine the feel and flow of the existing downstairs spaces. Open-plan living is popular for families. A large kitchen with a dining area is ideal for entertaining visitors. It is also a chance to hide any unsightly utilities and remove any unused space taken up by hallways and corridors.

Upstairs Interior Layout:

single brick rear house extension

Going for an extra storey with your extension gives you a chance to switch up the upstairs layout too. Of course, more bedrooms and bathrooms add value to your property, but you also can create something especially for you and your family. A master-suite? An office? An art studio? Extending upstairs can help you make room for your possessions, hobbies, and career all under one roof.

Types of Double Storey Extension

There are a variety of double storey extension styles. Here is a helpful list of the options:

Rear Extension

No matter what type of house you live in, detached, semi-detached or even a terrace, a rear extension is a popular choice. This type of extension is added to the back of the house only and takes up land at the back of the property.

Side Return Extension

If you want to keep the size of your back garden and you have space to the side, then this is a great option for you. There are some caveats though. Neighbouring properties may have overlooking windows which could limit the design of this type of extension.

Wrap Around Extension

Sometimes called an L-shaped extension, this type of extension adds space to both the rear and side of the property. If you want to maximise your interior and you have the land, then this is an ideal solution.

Front Extension

Adding to the front of your property is less common but it could be a possibility for you, especially if you have a large front garden or driveway. This type of extension can transform the overall appearance of your home. Whilst there are lots of design options, as this is the most publicly visible part of your property, there are stricter regulations to consider when getting planning permission.

The Best Double Extension for You

The simplest option, if you have a big enough back garden, is to go for a rear extension. As the most common type of extension, the planning and design process is usually smoother. If you do not want to lose any garden space, then a side extension could work. Your neighbours will require some consideration. That said, they may love the idea of extending and want to file a joint planning application to use the space between your properties. This can save you both time and money.

If you are in the lucky position of having plenty of outdoor space and therefore more extending possibilities, then an architect can help. They will explain what your options are and be able to give you an insight into the timescales, planning and costs of your specific project. If outdoor space is tight, an architect will be able to outline what is achievable in a limited area.

Double Storey Extension Design - Exterior Materials

decorative flower on the floor next to a step

Any extension is going to dramatically alter the look of the property. Planning permission may be granted more readily if you try to blend the extension into the look of the existing property. For example, matching brick, render or cladding. However, there is the possibility of pushing the design envelope and creating something unique. Combining striking modern architecture with period features can have a dramatic impact. Visualising the finished result can be difficult and making a design choice that you end up not liking can be costly to rectify. Consulting an architect or a designer to create mock-ups can help you decide with confidence.

Double Storey Extension Design - Doors and windows

You will lose some existing doors and windows to make way for the new extension. When designing the extension, considering openings for doors and windows is important. You need light for function, but it is also a significant design element. You can choose doors and windows to be in keeping with existing ones. Or you can use the opportunity to install something different. For example, sliding, pocket or bi-folding doors. To maximise light, think about skylights, roof lights, glazed walls, and structural glass.

Double Storey Extension Design - Roof

What your roof will look like once you complete the project is subject to planning requirements. There are still options to consider. A roof that ties in with the existing property might suit you best. If you wish to contrast the extension with the existing house, you can opt for a green or living roof. An architect can make you aware of all the options available to you.

Double Storey Extension Design - Indoor/Outdoor Living

You can maximise the impact of your extension by incorporating your outdoor areas. Sliding or bi-folding doors that open onto a patio, or green space can add an element of bringing the outside in and allow you to enjoy your garden more.


Calculating the cost of the project before you start is essential. How much is a double extension likely to cost? Each project is different and will have a range of factors contributing to the overall cost. The size, layout, fixtures, and fittings will have a bearing on the budget. As will any complications with the site. For example, if you plan to build on sloping ground, this will be more costly than ground that is flat because of the additional engineering costs.

The Average Cost of a Double Storey Extension

In the UK, the cost of adding a double extension to your property varies. It ranges from £1500 to £2500 per square metre, excluding finishes. In London, this range increases from £1800 to £2800 per square metre, excluding finishes.

Each additional storey will cost approximately 75% of the cost of a single storey.

Here is a handy table to demonstrate how the specifications of a project (the quality of the build fixtures and fittings) alters the budget. These figures exclude the cost of finishes.

Average Cost of a Single and Double Storey Extension in London per square metre (before finishes)

Spec Single Storey Extension Double Storey Extension
Basic £1800 £1350
Medium £2100 £1575
High £2800 £2100

There are other fees to think about. These include design fees, engineer's fees, planning application fees, building regulation application fees and party wall survey fees. These fees will usually add £1900 - £3200 to the cost of the build. It is also worth bearing in mind that all works will be subject to 20% VAT.

modern residential room

An Example of a London Double Storey Extension

Below is a table to shows an example of the costs of a double storey extension in London. For this example, the size is 4m by 5m giving a total of 40square metres of additional space over two storeys.

Typical Costs of a Double Storey Extension adding 40sqm

Basic Spec £54,000
Medium Spec £63,000
High Spec £84,000
Refurbishing Adjoining areas £30,000- £90,000
Other Fees and VAT £15,000 - £40,000

A Double Storey Extension Adds Value to Your Home

The costs are significant, but the return on investment can be significant too. Adding space adds to the asking price a property can demand. Even in the cheaper boroughs of London, the value of additional space is more than £4,000 per square metre. When compared to the cost of a double extension (at the high spec end) of £2,800, channelling funds into improving your home rather than moving home still makes financial sense. In the more exclusive areas of London, adding square footage and be extremely profitable.

Of course, not all value is financial. The added space can improve your living environment, adding comfort and reflecting your personal style.

Complex Building Site

contempurary living room

You may be working with a building site that has some constraints. If this is the case, allow room in your budget for extra costs. Issues can include:

  • The type of soil at the site. Clay, sand, composite ground, and marshy ground will all require the expertise of an engineer
  • The need for any structural changes to existing walls
  • The relocation of any underground infrastructure like telephone services, pipework, drains
  • The relocation of any utility supply devices like a gas meter and electricity meter

Double Storey Extension - Fees to Consider

Along with building costs, there are other fees to consider. When added up, these fees can account for a sizable chunk of cash.

Designing Fees

Getting professional help with the design will help your extension dreams become a reality. Focusing on the end look and practicality of the project but also on smoothing the process through the planning permission and building regulation approval processes. Fees can vary and alter your budget by thousands of pounds.

Architect Fees

Hiring an architect usually accounts for 7 - 15 % of the construction value in London, depending on who you hire. Double storey extensions require building regulation drawings by an architect or a design consultant. You can expect to pay between £3,000 and £5,000 for these drawings.

A cost-effective option is to hire a design and build company, like Proficiency Bespoke. Our quotes include all the design fees.

Structural Engineer Costs

A structural engineer is responsible for calculating the weight of a planned structure and incorporating the need for any structural elements into the build. The engineer will determine the size and depth of the foundations and if any reinforcements are needed. This would also include the need for the right type of steel beam and any other supporting aspects of the build.

For a double storey extension, a structural engineer is essential. Their fees will depend on the size of the build and the complexity of the site. You can expect to pay between £900 and £1,800 for an engineer working on a double storey extension in London.

Double Storey Extension Planning Costs

Open plan living room

We will discuss the ins and outs of the planning process later in the guide. For costing, here is a list of planning fees you may encounter:

  • Certificate of Lawfulness £100
  • Planning Permission Application around £200
  • Planning Condition Discharge £34 each
  • Party Wall Agreement Fees between £700 and £1500 per neighbour
  • Building Control Certificate around £750
  • Survey of existing building £500 - £1500
  • Tree report £250
  • Flood Risk Assessment £250
  • Ecology Report £400

There are other fees if you live in a listed building, in a conservation area or on a site of historical significance. These fees can vary depending on your site, if planning permission requires further reports and who you hire to provide these reports. Your local council can advise you on what your site requires.

Double Storey Extension Fitting Costs

Fitting costs can vary depending on what rooms you are including in the addition. For example, living space is cheaper to fit out than a new kitchen. Below is a cost guide to help you understand the potential price of what you have in mind.


Dry Lining and Plastering

A skim coat of plaster, for existing walls, is around £22 per square metre. For new walls, you will need both drylining and plaster skimming which will set you back around £47 per square metre. This increases to £50 for the ceilings. Costs also increase for older or listed properties that use clay or lime plaster. Some jobs will require scaffolding which will also bump up the price.


The quality of the paint and the experience of the painter vary the price here, but you can expect to pay £60 - £80 per square metre.


Supply and installation of the floor will cost around £50 - £150 per square metre, again depending on the specification of the flooring you choose.


A larger space will require additional heating to feel comfortable. Here are some options:

Adding on to your existing heating system - A plumber can carry out this work for you. It will usually take a day or two and will cost a minimum of £1,000

Underfloor heating - This is a more expensive, but more luxurious approach. It ups the comfort factor in large open plan extensions. Prices range from £70 - £130 per square metre. You can either choose electric underfloor heating or water-fed, electric being the cheaper option of the two.

New boiler - If you are extending your property significantly your current heating system may need a boiler upgrade to manage the added demand. This will cost between £2,500 - £6,000.


The sky is the limit if you are installing a new kitchen. A reasonable minimum budget would be around £8,000. But if you have the finest finishes in mind with high-end appliances then you can pay as much as £75,000.


Adding bi-fold or sliding doors can connect your home to your outdoor spaces. These will cost between £1,500 - £3,000 per square metre.


If you are adding or replacing windows, then you are looking at a cost of £500 to £3,200 per window depending on the specifications.

Bathroom or Shower room

Again, the price varies depending on the specification and you can expect to pay a minimum of £4,500 going up to £11,000 or more.

large home extension


Gaining planning permission can be a considerable obstacle in getting home renovations completed. Some double storey extensions will need planning permission, but some will not. These additions are known as Permitted Developments. It is often quicker, and therefore cheaper, to extend under permitted development but there are some strict rules, and the scope of the project will be more limited. We have outlined the ins and outs of planning below:

Permitted Development

To remove the need for gaining planning permission, your double storey project must:

  • be less than 50% of the free area around your home,
  • be less than 50% of the width of your home,
  • be a length of fewer than 4 metres for a detached and 3 metres for a semi-detached house,
  • not exceed the height of your home,
  • have a distance of at least 2 metres between the extension and the property boundary,
  • visually blend with the appearance of the existing house and the other houses in the neighbourhood, and
  • not include balconies or terraces.

If your plan meets all the above criteria, you will not need to apply for planning permission. However, it is still foresighted to acquire a Certificate of Lawful Development from your local council once the project is completed. This certificate verifies that you have completed the extension within the constraints of permitted development.

Open plan living room

Planning Permission

You will require planning permission if:

  • the extension takes up over half the area of the land around your house,
  • the extension is towards a road,
  • the extension exceeds the height of the existing building,
  • you plan to extend further than 6 metres from the rear of a semi-detached or terraced house,
  • you plan to extend further than 8 metres from the rear of a detached house,
  • the extension is taller than 4 metres,
  • the extension covers more than half the width of your existing home,
  • you plan to use materials that differ from the existing house, or
  • you plan to add a balcony or terrace.

An architect or design company can help you figure out the scope of what you can achieve with permitted development, or they can draw up plans most likely to achieve planning permission.

Party Wall Agreements

If you plan on building near, digging foundations near or altering a wall that you share with a neighbouring property then you need a Party Wall Agreement. These can add time and cost to the project.

Abiding by Building Regulations

All work done to extend a property must conform with building regulations and will require a certificate to demonstrate compliance. A professional building company can consult with local building control officers during the work and obtain the building regulation certificate at the end of the project.


Timescales for double storey extensions vary and many schedule disrupters can be outside of the homeowner's control. Below is a helpful list of timeframes to keep in mind when planning your extension. At Proficiency Bespoke we can help get your project completed faster as we work on various aspects of the process simultaneously.

house extension
  • Design and Planning: - 8 - 12 weeks
  • Party Wall Agreements: - 2 weeks - Several months. This depends on if negotiations go smoothly. Having a good relationship with your neighbours can help.
  • Technical Drawings, Building Regulation Approval, Structural Design: - 4 – 8 weeks
  • Construction: - 14 - 16 weeks

We Can Help

This expert guide may have given you the motivation and confidence to start thinking about a double storey extension, but the process can still feel overwhelming. We can help. At Proficiency Bespoke we draw from over a decade of experience in the home improvement industry and can guide you through the process from the design and the construction to installation and interiors. Our expertise touches every step of the project, resulting in an exceptional extension.

We approach each project by listening first. We want to understand your wishes and dreams to inform our process each step of the way. We are committed to you and your vision. We ensure that we respect your property and your neighbours from the start of the process through to completion. We will endeavour to:

  • Minimise disruption
  • Adhere to schedules
  • Stay within budget
  • Lessen the stress
  • Provide client satisfaction

Let our experience and expertise ease the extension of your house, so it can become somewhere you are proud to call home.