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Planning and Building a 3 Meter House Extension

3m Rear Extensions | UK-wide | Building Control Ready

Add 3 metres of space. Get Building Control approval without the stress.

We design safe, compliant rear extensions across the UK. Fixed fee, fast turnaround, full structural calculations and RSJ / steel beam design for Building Control sign-off.

Get my fixed fee quote

Single storey rear extension under construction in the UK

Building Control comment

Opening is 3.2m wide. No beam size provided. No padstone detail. Submit structural calculations.

We handle this,
not you.

Your 3m extension plan (zero panic)

Most homeowners feel like this before starting a 3m rear extension:

Will the council block it? Do I need planning? Can I remove that wall safely? I can’t afford to get this wrong

breathe
this is fixable

What actually happens:

We check if your 3 metre rear extension fits permitted development. We size the RSJ for the opening when you knock out the back wall. We give you padstone and foundation notes for Building Control. This is our everyday work: home extension Building Control support .

Rear wall removed → beam carries the load

this beam + padstones
gets signed off

SECalcs

Adding extra living space to your home and raising its value is a popular choice among homeowners, and a home extension is an excellent way to achieve this. You can opt for a single-storey extension or a loft conversion, depending on your needs. In the UK, kitchen extensions are a popular choice since they provide more functional space for cooking and entertaining guests. Loft conversions are also an option that can transform your unused attic into a comfortable bedroom or office. For more on successfully converting lofts: loft conversion guide . If you require additional living space within a limited budget, then a single-storey extension might be the right choice. It can be added to the side or rear of your property and provides up to 3 meters of extra space. This type of extension is ideal for those who do not want the hassle and expense associated with two-storey extensions. Before deciding on the type of extension, it’s important to consider your budget, requirements, planning permission, and building regulations. Hiring an experienced architect or contractor can help you make informed decisions about which kind of extension would suit your needs best. With careful consideration and proper planning, a home extension can transform your living space and add value to your property. We cover this in more depth here: home extension structural design .

Contemporary rear / side extension opening out to the garden.

Kitchen diner style extension in progress

Turning two small rooms into one bright kitchen / diner.

Finished extension and upgraded roofline in the UK

Extension adding liveable space without moving.

Benefits Of Extending Your Home

Why a 3 metre rear extension or kitchen extension is still one of the best upgrades you can do. See more about extensions: house extension advice.

Space

Additional Living Space

Expanding your home is an ideal solution for growing families, home offices, or hobbies. A 3 meter expansion can provide a new bedroom, bathroom, or living room, giving your family more space to enjoy.

Value

Increased Property Value

Extending your home can significantly boost its value and make it a wise investment for the future. Even a 3m single-storey extension can lift resale appeal.

Design

Personalised Layout

You choose how the new space works: rear, side or wrap-around. Glazing, insulation, heating, layout. A tailored space for how you live.

Control

Cost-Effective Space

A single-storey extension is often cheaper and less disruptive than moving. You can phase the work, control finishes, and stay in the house you love.

Rear kitchen extension with full-height glazing.

Surveys, measurements and structural design before you spend.

Additional Living Space: Expanding your home is an ideal solution for growing families, home offices, or hobbies. A 3 meter expansion can provide a new bedroom, bathroom, or living room, giving your family more space to enjoy.

Increased Property Value: Extending your home can significantly boost its value, adding much value and making it a wise investment for the future. Whether you live in a detached property or a semi in a conservation area, an expansion of 3 meters or more can greatly enhance your home’s worth.

Personalized Design: Extending your home allows you to personalize and create a space that meets your unique needs and preferences. With plenty of design options available, such as rear, side, or wrap-around extensions, you can choose the best fit for your home. You can also select glazing options, insulation, and heating to make your extension comfortable and energy-efficient.

Cost-Effective: Expanding your home is a budget-friendly way to add extra living space and increase your property's value. By using an extension cost calculator, you can estimate the project cost and plan accordingly. You can also save on labor costs by taking on some of the work yourself if you have the skills.

Before planning your home extension, keep in mind that you may require planning permission and need to notify your neighbors if the extension involves work on a party wall. Additionally, make sure your extension meets the width and square meter requirements in your area. With the help of a surveyor and a detailed specification, you can ensure that your extension is built to the highest standards.

Planning Permission And Building Regulations

Planning is about what you're allowed to build. Building Control is about how safely it’s built. For official national guidance see the Planning Portal.

Planning Permission / Permitted Development

In most cases, a 3 meter addition to your house falls under permitted development rights, which means you don’t need full planning permission as long as it meets certain criteria:

The addition cannot be higher than the highest part of the existing roof.
For an attached house, you cannot extend more than 3m beyond the original rear wall. For a detached house, 4m.
You cannot cover more than 50% of the total area of land around the original house.

If your addition does not meet these criteria, you will need to apply for planning permission by submitting a planning application.

3m rear on a semi
is usually fine

Building Regulations / Safety + Structure

Even if the extension is permitted development, it must still comply with Building Regulations. That means the structure, insulation, foundations and beams all have to meet the standard. We provide that pack: structural engineers for Building Control .

Submit a building notice or full plans to the council / approved inspector
Show foundations, walls, doors, insulation, roof build-up
Provide structural calculations for any RSJ / opening in load-bearing walls
Get sign-off at each stage of the build

This is where
SECalcs steps in

Surveyor on site for UK house extension compliance

On-site checks, foundation depth, bearings for steels, insulation and fire safety are all part of Building Control sign-off.

In most cases, a 3 meter addition to your house falls under permitted development rights, which means you don’t need planning permission as long as it meets certain criteria. These criteria include:

The addition cannot be higher than the highest part of the existing roof.
For an attached house, the addition cannot extend beyond the rear wall of the original house by more than 3 meters, and for a detached house, it cannot extend beyond 4 meters.
The addition cannot cover more than 50% of the total area of land around the original house.

If your addition does not meet these criteria, you will need to apply for planning permission by submitting a planning application.

Building Regulations For A 3 Meter House Extension:

Even if your 3 meter house extension falls under permitted development rights, it still needs to comply with building regulations. These regulations ensure that your extension meets safety, energy efficiency, and accessibility standards.

To comply with building regulations, you’ll need to:

Submit a building notice or full plans application to your local authority
Ensure that the foundations, walls, doors, and windows of your extension meet the appropriate standards
Get approval from your local authority building control officer at key stages of the construction process

Cost, Design and What Affects Price

Where the money actually goes on a 3 metre single storey rear extension.

Step 1

Measure & Layout

You decide how far you’re going out and how wide the opening will be. Bigger opening = bigger beam. We’ll tell you what’s realistic: extension structural design service .

Step 2

Structural Design

We size RSJs, padstones and check foundations. This is what Building Control wants. Learn more: steel beam design for extensions .

Step 3

Build Cost

Most 3m rear extensions land around £25,000 – £45,000+, depending on glazing, rooflights, doors, drainage and finish.

Step 4

Approval & Sign-Off

Building Control signs each stage. Our calcs go in your file so you’re covered when you sell the property. This protects future buyers and protects you.

Finished rear extension with full-width glazed doors.

Structural opening created for extension

Old rear wall removed, RSJ installed, space opened.

Exemptions And Limitations For Certain Types Of Extensions:

Different types of house extensions, including kitchen, side return, and wraparound variations, may have distinct rules and limitations that differ from standard extensions. These unique extensions can significantly increase the value of your home, particularly in London.

For example, conservatories or orangeries may be exempt from building regulations if they satisfy specific criteria, such as being separated from the main house by external-quality doors and windows. To ensure that your addition is energy-efficient and comfortable, you will need to consider factors such as flooring, insulation, glazing, and heating.

Likewise, porches may also be exempt from building regulations if they meet certain size and height requirements. However, it’s important to check with your local council before beginning construction to see if planning permission is required or if your property is located in a conservation area.

Understanding the building regulations and planning permission requirements for your 3 meter house addition is essential to ensure that your project is safe, legal, and meets the appropriate standards. By familiarizing yourself with these requirements and limitations for specific types of additions, you can avoid potential problems and ensure a successful project. You can also use an online cost calculator or consult with a surveyor to estimate the costs and create a checklist of building work that needs to be done.

Design Options

When it comes to designing your 3 meter house extension, there are several options to consider, including:

Single storey extension: Extend the ground floor for a new living area, kitchen, or dining room.
Two-storey extension: Extend both ground and first floor for extra bedrooms/bathrooms.
Wrap around extension: Extend along the rear and side to create open-plan kitchen/diner + utility/home office.

Factors To Consider When Choosing A Design:

Your budget: Design choices (rooflights, doors, glazing) affect cost.
Your property type: The extension should look like it belongs to your house, not stuck on.
Your needs: Are you chasing space, resale value, a home office, or an open kitchen/diner?

Popular Design Trends:

Open-plan living: Kitchen / dining / seating all in one bright space.
Bi-folding doors: Garden connection, lots of light.
Minimalist detail: Clean lines, hidden structure, uncluttered surfaces.

Cost Estimates

The cost of a 3 meter house extension can vary depending on several factors, including:

Size of the extension
Design complexity (roof lanterns, corner glazing, etc.)
Materials (standard brick/block vs high-end finishes)
Labour / contractor rates in your area

According to industry estimates, a 3 meter house extension can cost between £20,000 and £50,000, depending on those factors.

Factors That Influence The Cost:

Planning permission (if required)
Building regulations compliance (structural calcs, insulation spec)
Site access and complexity
Unexpected issues like drainage, services, old foundations

Cost-Saving Tips:

Keep the design simple instead of lots of structural breaks and weird angles.
Pick sensible materials rather than bespoke glass everywhere.
Get multiple quotes.
Plan ahead so you’re not changing the design mid-build.

A 3 meter house extension can be a significant investment, but it can also add significant value to your home. By understanding the factors that influence the cost and implementing cost-saving tips, you can keep control of your budget.

Hiring A Contractor

Research potential contractors: Look for people with actual rear extension experience. Check references and see finished work in person if you can.

Verify insurance: You want them insured. Full stop.

Get multiple quotes: If one is suspiciously cheap, there is a reason.

Read the contract: Timeline, payment stages, who handles Building Control communication.

Questions To Ask:

Have you done 3m rear extensions before?
Who’s installing the steel and padstones?
Have you worked with Building Control in this council?
Can I see recent photos?

Red Flags:

No insurance
High-pressure tactics
Won’t show previous work
Vague about “who’s doing the calcs”

Recap Of Key Points

Extending your house by 3 meters can be an excellent way to increase the value of your property and enhance your living space. The key is planning clearly:

Know if you’re inside permitted development or you need planning
Get proper structural calculations for any opening / RSJ
Follow Building Regulations for foundations, insulation, and support
Hire someone who’s actually done this, not someone guessing steel sizes

By following these steps, you can get the extra living space you want, protect the value of your home, and keep Building Control happy first time. If you want expert drawings and calcs: speak to SECalcs.

3 metre house extensions – FAQs (2025)

Planning rules, cost and Building Control

These are the most common questions we get from homeowners in Greater Manchester about 3m rear extensions, single storey kitchen extensions and wrap-around side returns.

Do I need planning permission for a 3 metre rear extension?

Even if I don’t need planning, do I still need Building Control approval?

How much does a 3m single storey extension cost in 2025?

Will I need RSJ / steel beams when I open up the back of the house?

Do I need party wall notices for a 3m extension?

What drawings and calcs do Building Control actually ask for?

How fast can I get structural calculations for my extension?

Can I manage the build myself to save money?

Will a 3m rear extension add value to my house?

What do you actually do for me as structural engineers?

Home extensions are like larger suitcases for travel, providing ample space for growing families, home offices, or hobbies. They enhance property value and offer customized designs, but it’s important to consider budget, planning permission, and building regulations beforehand.

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