Thinking about extending your home in Hertfordshire? Whether you live in Harpenden, St Albans or anywhere across Hertfordshire, a side or rear extension is one of the best ways to add space, value and functionality to your home, without the stress of moving.

The good news is that many home extensions in Hertfordshire can be built under Permitted Development (PD) rights, meaning you may not need full planning permission at all. But the rules can be confusing and getting them wrong can be costly.

This 2026 guide breaks down exactly what you need to know: what is allowed under Permitted Development, what the size limits are, when you do need planning permission, and what the Prior Approval process involves. We also cover common issues homeowners in Harpenden, St Albans and Hertfordshire face when planning side and rear extensions.

What Is Permitted Development for Home Extensions?

Permitted Development rights are a set of rules granted by the government that allow homeowners to carry out certain types of building work without needing to apply for full planning permission. These rights are set out in the Town and Country Planning (General Permitted Development) (England) Order 2015, with updates that apply in 2026.

For homeowners in Hertfordshire, this is great news. It means that under the right conditions, you can build a rear extension, a side extension, or even a wrap-around extension without going through the full planning application process.

However, Permitted Development rights do not apply to all properties. Flats, maisonettes, listed buildings, and properties in certain designated areas may have restricted or removed PD rights. Always check with your local planning authority before starting work.

Permitted Development Rules for Rear Extensions in Hertfordshire (2026)

Rear extensions are the most common type of home extension in Harpenden and St Albans. Under Permitted Development, the rules for rear extensions in 2026 are as follows:

Single-Storey Rear Extensions

• Maximum depth of 3 metres for an attached (terraced or semi-detached) house
• Maximum depth of 4 metres for a detached house
• Maximum eaves height of 3 metres if within 2 metres of the boundary
• Overall maximum height of 4 metres
• Materials must be similar in appearance to the existing house

Larger Single-Storey Rear Extensions (Prior Approval Required)

Under the Householder Prior Approval process, you can extend beyond the standard limits:

• Up to 6 metres depth for an attached house
• Up to 8 metres depth for a detached house
• You must notify the council and neighbours are consulted
• The council has 42 days to raise any objections
• If no objection is raised, work can proceed

Two-Storey Rear Extensions

• Maximum depth of 3 metres from the rear wall
• Must be at least 7 metres from the rear boundary
• Maximum eaves height of 3 metres (if within 2 metres of a boundary)
• No balconies or raised platforms above ground floor
• Roof pitch should match the existing house where possible

Permitted Development Rules for Side Extensions in Hertfordshire (2026)

Side extensions are also popular across Hertfordshire, especially where homes have a side garage or passageway. Under Permitted Development, the rules for side extensions are:

• Maximum width of 50% of the original house width
• Single storey only under PD rights
• Maximum eaves height of 3 metres if within 2 metres of the boundary
• Maximum overall height of 4 metres
• Cannot extend forward of the principal elevation
• Materials must match the existing property

Note: Two-storey side extensions will almost always require full planning permission. If you are considering a larger side extension or one that wraps around the rear of your property, you will need to speak to a planning expert or your local authority.

What Is the Prior Approval Process for Extensions?

The Householder Prior Approval (also called the Neighbour Consultation Scheme) allows you to build a larger single-storey rear extension than the standard PD limits. Here is how the process works in Hertfordshire:

1. Submit a Prior Approval application to your local council (St Albans City and District Council or Dacorum Borough Council, depending on your area)
2. Include a site plan, block plan and description of works
3. Your neighbours are formally consulted for 21 days
4. The council considers any objections based on impact, not general design
5. If approved (or if no decision is made within 42 days), work can proceed

At ACR Build, we regularly help homeowners in Harpenden and St Albans navigate the Prior Approval process. It is simpler than a full planning application, but it still needs to be done correctly to avoid problems later.

When Do You Need Full Planning Permission for an Extension?

You will need to apply for full planning permission for your extension in Hertfordshire if:

• Your property is a flat or maisonette
• Your home is a listed building
• You are in a designated area such as a Conservation Area or Area of Outstanding Natural Beauty (AONB)
• Your Permitted Development rights have been removed by an Article 4 Direction
• The extension exceeds the size or height limits for PD
• You want a two-storey side extension
• The extension extends beyond the principal elevation (front of the house)

St Albans has a significant number of Conservation Areas and listed buildings, so it is worth checking your property status before assuming PD rights apply. Our team at ACR Build can help you check this as part of the early planning stages.

Do You Need Building Regulations Approval?

Yes. Even if your extension qualifies under Permitted Development, you will almost certainly need Building Regulations approval. This is separate from planning permission and ensures that the work is structurally sound and meets current safety and energy efficiency standards.

Building Regulations cover:

• Structural integrity (foundations, walls, roof)
• Fire safety
• Damp proofing and drainage
• Thermal performance and energy efficiency (Part L)
• Electrical and plumbing installations

ACR Build works with structural engineers and submits all required Building Regulations applications as part of our full project management service. You do not need to chase this yourself.

Common Extension Types in Harpenden and St Albans

Over many years building extensions across Hertfordshire, these are the most popular types of extensions we carry out:

Kitchen Extension / Kitchen-Diner Extension

One of the most popular home improvements in Harpenden and St Albans. A rear kitchen extension can transform a cramped kitchen into a large open-plan kitchen-diner with bifold or sliding doors opening onto the garden. These typically fall within PD limits for single-storey rear extensions.

Wrap-Around Extension

A wrap-around combines a rear and side extension to create a large L-shaped addition to the back and side of the house. Depending on the size, these may need full planning permission, but can dramatically increase your living space.

Double-Storey Extension

Adding two storeys to the rear of your home can add a new bedroom and bathroom above a new ground floor living space. Two-storey rear extensions can fall within PD rights under certain conditions but require careful planning.

Side Return Extension

Very popular on Victorian terraced houses in St Albans and Harpenden, a side return extension fills in the narrow passageway to the side of the kitchen, creating a wider, brighter space. These often fall within PD rights as a single-storey side extension.

How Much Does a Side or Rear Extension Cost in Hertfordshire?

Costs will vary depending on the size, specification and complexity of your extension. As a general guide for Hertfordshire in 2026:

• Single-storey rear extension (3-4m): from around £45,000 to £80,000+
• Larger single-storey extension (5-6m): from around £65,000 to £100,000+
• Two-storey rear extension: from around £90,000 to £150,000+
• Wrap-around extension: from around £80,000 to £160,000+

These are approximate ranges. At ACR Build, we provide a detailed, transparent tender so you know exactly what you are paying for before any work begins. We strongly recommend getting a full itemised quote rather than a rough estimate, so there are no surprises along the way.

How Long Does a Home Extension Take in Hertfordshire?

Build times will depend on the scale of the project. For a typical single-storey rear or side extension in Harpenden or St Albans:

• Planning and design stage: 4 to 12 weeks
• Prior Approval or planning permission (if required): 6 to 12 weeks
• Build time (single storey): 8 to 16 weeks
• Build time (two-storey): 16 to 24 weeks

ACR Build works to a clearly agreed timeline from the start. Our director Andy Ryder is personally involved in every project, and we hold monthly on-site review meetings with clients to keep everything on track.

Permitted Development Checklist for Homeowners in Hertfordshire

Before starting your extension project, use this quick checklist:

1. Check if your property has full Permitted Development rights (not a flat, listed building or in a restricted area)
2. Measure your proposed extension against the PD size limits
3. Confirm whether you need Prior Approval for a larger rear extension
4. Check if full planning permission is required (two-storey side, over-limit sizes, restricted area)
5. Apply for Building Regulations approval
6. Appoint a reputable, experienced local builder
7. Consider a Lawful Development Certificate for peace of mind and resale value

Do You Need a Lawful Development Certificate?

A Lawful Development Certificate (LDC) is not required by law, but it is strongly recommended. It is a formal confirmation from your local council that your extension is lawful under Permitted Development. This is useful for:

• Reassuring buyers and mortgage lenders when you sell your home
• Providing a clear paper trail that the work was carried out correctly
• Protecting yourself if the council ever questions the development

We always advise homeowners in Harpenden and St Albans to apply for an LDC alongside their extension, even when it is not strictly required.

Why Choose ACR Build for Your Extension in Hertfordshire?

ACR Build is a Hertfordshire-based building company with over 20 years of experience delivering home extensions, renovations and new builds across Harpenden, St Albans and the surrounding areas. We are members of the Federation of Master Builders (FMB) and have won Best of Houzz awards in 2022, 2023 and 2024.

Here is what makes us different:

• Our director Andy Ryder visits every client in person from day one
• We provide a fully itemised, transparent tender with no hidden costs
• We manage the entire process: design, planning, building regulations, build and finish
• We work with trusted local architects, structural engineers and tradespeople
• We have dozens of five-star reviews from homeowners across Hertfordshire
• We deliver on time and within the agreed budget

We understand that an extension is a big commitment, and we make sure the experience is as smooth and reassuring as possible, from first conversation to final finish.

Frequently Asked Questions About Extensions Under Permitted Development in Hertfordshire

Can I build a rear extension without planning permission in Hertfordshire?

Yes, in most cases. Under Permitted Development rules, a single-storey rear extension of up to 3 metres (attached house) or 4 metres (detached house) can be built without full planning permission. For larger extensions up to 6 or 8 metres, you can use the Prior Approval process.

Do I need planning permission for a side extension in St Albans?

A single-storey side extension may be permitted under PD rights if it is no more than half the width of the original house. However, if your property is in a Conservation Area or has had its PD rights removed, you will need planning permission. St Albans has many Conservation Areas, so it is worth checking first.

How far can I extend my house without planning permission in 2026?

Under standard PD rights, up to 3 metres to the rear for a terraced or semi-detached home, or 4 metres for a detached home. Under Prior Approval, this increases to 6 metres and 8 metres respectively.

What is the maximum height for a single-storey extension under Permitted Development?

The maximum height is 4 metres overall. If any part of the extension is within 2 metres of a boundary, the eaves height must not exceed 3 metres.

Do I need Building Regulations for an extension under Permitted Development?

Yes. Building Regulations are almost always required for any extension, regardless of whether you need planning permission. Building Regulations cover the structural, fire safety and energy efficiency standards of the work.

How much does an extension cost in Harpenden or St Albans?

Costs start from around £45,000 for a straightforward single-storey rear extension, depending on size and specification. ACR Build provides a full, itemised tender so you have a clear picture of all costs before committing.

Ready to Plan Your Extension in Hertfordshire?

If you are thinking about a side or rear extension at your home in Harpenden, St Albans or anywhere across Hertfordshire, we would love to hear from you.

ACR Build offers a initial consultation with our director Andy Ryder, who will visit you at home, discuss your vision and help you understand what is possible within your budget and within Permitted Development rules.

Get in touch today:

• Phone: 07534 864 783
• Email: info@acrbuild.co.uk
• Website: www.acrbuild.co.uk
• Address: Unit 2A, Thrales End Farm Business Centre, Thrales End Lane, Harpenden, Hertfordshire, AL5 3NS

ACR Build. Trusted builders in Harpenden, St Albans and Hertfordshire for over 20 years.

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