Trimble Architects

RIBA Architectural Services

Phone Number: 020 8538 9303

Mobile Number: 07785711635

What Is The Process For Building An Extension

  • Posted by:
  • Admin
  • Tags:
  • Posted date:
  • 25-07-2022
What Is The Process For Building An Extension

What is the process for building an extension? This article looks at the process for building an extension and where to start with the building project.

Bob Trimble is a chartered architect registered with RIBA with 30 years experience in the industry. Trimble Architects work throughout Hounslow, Twickenham, Richmond, Kingston Upon Thames, Teddington and the surrounding areas of London.

Building an extension on a house – where to start

Planning and building a home extension project can be a big task, even for small extension projects, yet the result is often worth the difficulties. You can increase your already beloved space with an extension without having to move your belongings and deal with hefty moving fees. You can build a home extension to support a number of lifestyle changes, from a growing family to a new entrepreneur in need of home office space.

The addition of an extension not only increases the space in the home, but it can add value to the house for when you decide to sell. Value-added can be determined by the type of extension built and the best-selling features in the local area. Before choosing the type of extension, consider the fact that some are feasible under your 'Permitted Development Rights.

This means planning permission is not required, reducing timescales and reducing costs for the project. Further comprehensive information regarding these rights can be found online with the UK Government's official guide: 'Permitted Development Right for Householders Techincal Guidance'.

An example would be Loft Conversions; this type of home improvement generally doesn't require any planning permissions, unless the height or width of the loft is extended further than the original roof outline.  However, we offer a condensed and equally informative list below, as the official guide is quite lengthy. Below we discuss which extensions fall under this category and the conditions. 


What size extension can I build without planning permission?

Single-storey rear extension 

Under your right for Permitted Development, you can add a single-storey extension to the rear of your home under two specific circumstances;

  • Height must not exceed 4m
  • The width must not exceed 4m (detached) from the original rear wall, or 3m (non-detached, terraced)

Moreover, if the new extension is being added to an existing extension, then the above measurement guidelines should include both the old and new builds. Using the 'Neighbour Consultation Scheme,' you can even push these limits further with non-detached houses allowing 6m width from original build and detached houses allowing 8m. This scheme means planning permission isn't required.

Instead, it allows you to make a proposal and give your neighbours 21 days to object to the idea, with valid reasoning to support them. Once this period is over, the Local Planning Authority will have another 21 days to grant approval and request a fee for a Lawful Development Certificate; this proves you don't need planning permission and your Permitted Development is legal. 

Two-storey rear extension  

Similarly, you can build a two-storey rear extension without planning permission, as long as it meets the following requirements;

  • Height must not exceed 4m
  • Distance from the rear wall must be no more than 3m
  • Should be within 7m from the boundary of your plot that's directly opposite that wall

Be cautious that adding another storey on top of an existing single-storey extension when the original build extends by more than 3m beyond the rear wall will require planning permissions. It will not be legal under your permitted development rights, so always check before planning begins. 

Side extension

A side extension can be built without planning permission if its width is no more than half of the width of the original house, single-storey and no higher than 4m. If the new extension is an add-on to a previous extension, then the above measurements apply to them both together. If the side extension is built to pass beyond the rear wall, whether or not it's attached to the wall, then it is subject to the same rules. 

  • Height must not exceed 4m (one-storey)
  • The width cannot exceed half of the width of the original house 

Rules may vary depending on the location; for instance, in Wales, a two-storey side extension may be possible under a Permitted Development application.

Rear-plus-side extension

If you want the extension to extend the rear and side walls, the restrictions become slightly more confusing. Ensure your design fits into requirements before any further steps are taken.

  • Height must not exceed 4m (one-storey)
  • No more than 6m past the rear wall (terraced, non-detached) or 8m (detached)
  • The width cannot exceed half of the width of the original house 

'Infill extensions' refer to those built in the space between the rear and side wall, often used to square off the house blueprint. This type of extension is allowed under Permitted Development Rights. You can also build two separate buildings, although wraparound designs require planning permission as they'll likely exceed width limits. 

Height limit planning rules

Height restrictions for extensions are quite straightforward. Simply put, the top height of the new extension should not exceed the height of the original home's highest roof ridge line or flat roof. The extension's height must not exceed the height of the highest roof ridge line (or flat roof) of the existing house.

Eaves are the edges of the roof that overhang the wall face to the side of the structure. Eaves are the edges of the roof which overhang the face of a wall and project beyond the side of a building. If the new extension is within 2m of the boundary line, the eaves must be no higher than 3m, according to the Permitted Development Guidelines.

Boundary restrictions

Whenever boundary restrictions are involved, it's best to have an open conversation with your neighbour to discuss the planned extension. Alternatively, you can speak to the local planning office, who will consult with your neighbour offering the relevant information.

The neighbour then has 21 days to reject the plans, giving fair reasoning, and then another 21 days to wait for the planning office to accept the project.For single-storey extensions, it cannot go beyond the rear boundary by more than 3m, and for double-storey, it is 7m.

These guidelines only apply if no other properties are on the land rear to yours. Side boundaries can be built on so long as this doesn't impact the neighbour's rights.

The elevation of the area should consider the 45° rule used by local planning authorities, which factors in the entry of sunlight and daylight onto the neighbouring properties.

What Is The Process For Building An Extension?

To check for this, find your neighbour's closest window and draw the imaginary line to see if it will breach. This also relates to the neighbours' Right to Light', which often comes under the Prescription Act 1832, or the more recent 'Rights to Light Act 1959.

Overall, its' best to be honest with your neighbours and show them the extension designs, especially when boundaries are involved. Some disagreements may arise, but you'll have plenty of time to adjust the designs and rectify the problem before it's too late! A general rule is; that if the extension will be any higher than 7.2ft, you should speak with your neighbour; any lower, and it should be okay.

Steps To Adding An Extension To Your Home

Creating more space in your home is becoming increasingly popular as the way we use our homes evolves. We now expect our homes to provide a place to eat, relax, work out, entertain and more; as demand increases, so does the need for usable space resulting in a need for extensions. As housing prices rise and the market worsens, there is an excess of demand that cannot be met.

In this volatile housing environment, many are opting for home improvements and house extensions. It's a very smart move to increase your existing space rather than opt for the challenges of securing and financing an entirely new home. Moreover, houses can hold a lot of important memories, and with an extension, you not only get the benefits of a new space, but you get to retain the home you cherish and add resale value to the home.

Below we discuss the route you should follow to construct a successful, efficient and feasible plan for a new home extension project. 

Step 1

Research, Research, Research!

If you're thinking of building an extension on your home, you should first head to your local estate agents to highlight the most popular type of home improvements that sell well; for instance, a P-shaped glass conservatory or a garage conversion may be more desired than a standard extension.

It would be best if you made sure the cost is worth it with the added value to your home exceeding the costs. Another way to gain invaluable advice is to speak to someone you know who's recently undergone a house extension or addition, they can suggest contractors and discuss the problems that arose for them. This can help you build a more full-proof plan and remain within your budget. 

step 2

Get that Planning Permission!

You can build some extensions without planning permission, thanks to your 'permitted developmental rights' (PDR). In 2008, planning laws were relaxed, introducing PDR, allowing an 8m single-storey extension to the property's rear to be built without planning permission. Similarly, double-storey extensions to the back can extend by 3m without planning permission.

Extensions to the side of the house can stretch out to half the width of the existing property and be no higher than 4m without the need for planning permission. Any taller or wider than this, and you'll need to contact your local planning office to begin the planning and design process.

If your home is in an area of nature conservation, you MUST talk to the local planning office before any extensin work begins, ideally before planning. Thanks to the internet, completing planning permission forms have gotten much easier, although you can still choose to use paper copies.

Online: Using the UK Government Planning Portal, you can complete the form with your relevant personal details and details of the proposed extension. A questionnaire is given, ensuring all the important questions are answered. Once completed, the forms are sent to your Local Planning Office to be reviewed.

Paper: Simply download the online forms from the GOV Planning Portal website and complete them by hand. These should then be sent by post to your Local Planning Office. 


step 3

Check out Building Regulations

No matter what extension you build, you must comply with all up-to-date building regulations. Whether you require planning permission or not, you must remain in government and locally set guidelines.

You can confirm building regulations were met in two ways, either by submitting a 'Full Plan Submission' to the local planning office and paying a building inspector to regularly check that progress meets regulations. Or by submitting a 'Building Notice' to the local council to highlight your intention to start building within 48 hours, whilst ensuring all regulations will be met. Professionals recommend you submit a Full Plan to local authorities for the least risk. 

step 4

Choose your Architect

Architects are very important for house extensions, they can help to ensure designs make logistical and structural sense, whilst providing a design aspect. Even with simple extensions hiring an Architect can prove useful as they make great project managers and provide expertise for complex designs.

To find a trusted Architect, you can contact the Royal Institution of Chartered Surveyors (RICS) or the Chartered Institute of Architectural Technologists (CIAT). Make sure you confirm their qualifications and professional indemnity insurance, as well as looking at reviews and previous work to ensure they match your vision, style and budget. 

step 5

Find a trusted Builder

Selecting a trusted and professional builder is vital as they help with the project management responsibilities. Unless you plan on managing the project yourself, which can be incredibly stressful and time-consuming, not to mention the lack of experience or knowledge, working with a build and design company provides many benefits.

Firstly the lead builder can manage the project with ease as they've got experience with the other builders, additionally, it can save you money from further external contracting. When selecting a builder or build and design company, the best place to start is by checking any personal recommendations and reviews.

Previous customers will give the most honest reviews, whether good or bad, offering information on work rates and working relationships, as well as identifying issues so you can avoid them! Don't just choose the first builder or build company your recommended; make sure you gather a handful of fixed quotations so you can easily compare.

With relevant information, you can select the most cost-effective and trusted choice. The builder must retain the relevant professional certifications, and they should be proud to show their previous works. Ensuring the builders or building companies have a good reputation is vital for the integrity of the build and the reliability of the contractors. 

step 6

The Big Move

Some extensions may be low impact with limited obstruction, whilst others may disrupt every part of your daily life. The number of people living in a home and whether it's a family unit can also affect the need to move out whilst construction takes place. For example, a new kitchen or front porch can affect morning school or work routines, so you may need to consider alternative routes.

In the worst case, you may need to move out during the most disruptive part of the process. Speak to family and friends who may be willing to help you out with temporary accommodation; alternatively, you could book local lodging.

Although it may seem like a good idea, NEVER go on holiday and leave the builders in charge without updates. Travelling during home improvements can lead to longer waits and lower-quality or rushed finishes, as well as the uncertainty when an issue arises due to lack of communication. 

Last word...

The options for a home extension are endless; whether you want to extend an existing room or create a whole new room, there'll be something to suit your needs and wants. Consider your budget and the planning permissions required for the design before you begin construction to avoid unwanted hold-ups.

And when construction does begin, ensure the team you've chosen is Trustworthy and Professional, meaning the final product will be top-quality and produce a high-quality finish. 

Here at Trimble Architects, we offer a non-obligation online consultation service. Simply input relevant personal details and details of the proposed extension into the online form. So what are you waiting for? Contact our trusted and professional team to start your home improvement journey today!

options for a home extension

 Architects Near Me


If you are looking for architectural services throughout Hounslow, Twickenham and the greater London area, then get in contact with our team today. We can offer advice and begin guiding you towards making the right decision.


Bob Trimble is a chartered architect registered with RIBA. Bob Trimble has 30 years of experience working with residential and commercial property projects. For 4 years, Bob Trimble has worked from his housing association and private architectural practice for clients throughout Hounslow, Twickenham, Richmond, Kingston Upon Thames, Teddington and the surrounding areas of London.