Your 2025 Guide to Double-Storey Extension

Is your home starting to feel a little cramped? Whether you’re welcoming a growing family, setting up a productive home office, or simply craving more room to enjoy, a double-storey extension could be the perfect way to transform your living space.

Rather than going through the hassle of moving, you can consider a double-storey extension allows you to expand without leaving your home. It’s a smart solution that gives you the extra space you need, all while keeping the things you love about your current home.

In this guide, we’ll walk you through everything  you need to while exploring double storey extension from planning regulations and selecting the right builder, to understanding the costs involved. Let’s explore how a double-storey extension can make your home work for you.

So What is Double Storey  Extension ?

A double-storey home extension is a two-tier addition to your house, where both an upper and lower floor are added to your existing structure. This could involve building an entirely new extension with two floors, or converting an existing single-storey extension into a more spacious, two-storey design.

This type of extension offers versatility, allowing you to add extra bedrooms upstairs while providing additional living or functional space on the ground floor—whether that’s a larger kitchen, extra bathroom, or any other room you need.

Now, let’s explore two ways you can achieve a double-storey extension: extending to the rear or extending to the side of your property.

1. Rear Extension

Extending to the rear is a popular choice for many homeowners, especially those with spacious back gardens. This approach uses the available garden space, which often makes it the most practical option for expanding your home. One of the key benefits of this type of extension is that it doesn’t alter your house’s facade, which is typically a positive factor when seeking planning permission from the council.

Additionally, many houses are wider than they are deep, so a rear double-storey extension can create a more square-shaped design, resulting in well-proportioned, neat rooms. This is ideal for those looking to expand their living area while maintaining the external appearance of the property.

2. Side Extension

For those not living in a terraced house, adding a double-storey extension to the side of your property is another option. However, this does require planning permission, as permitted development rights do not apply here. The advantage of a side extension is that it allows you to increase the overall width of your home, creating the space for a larger kitchen, living area, or additional bedrooms.

If you have enough space on one side of your property, a side extension could even enable you to create an entirely new wing, or in some cases, a separate dwelling. This option is particularly suitable if you’re looking to maximise your indoor space without encroaching too much on your outdoor area.

Double-Storey vs. Single-Storey Extensions: Which One Is Right for You?

If you’re looking to expand your home, a double-storey extension could be the perfect solution. Not only does it provide significantly more space, but it also allows for a more flexible and creative approach to your home’s layout. Whether you’re adding extra bedrooms, expanding your living areas, or designing an open-plan space, a double-storey extension can transform your home and enhance your lifestyle.

One of the biggest advantages of a double-storey extension is the long-term value it adds. By increasing your home’s square footage, you’re not only improving functionality but also boosting its market appeal. Should you decide to sell in the future, this investment can offer substantial financial returns.

While a double-storey extension is a more significant project, working with experienced professionals can ensure a smooth and well-managed process. From obtaining planning permission to designing a layout that maximises space and light, expert guidance can help bring your vision to life efficiently and effectively.

Single-storey extensions, while a great option for adding space, may not provide the same transformative impact. They remain a cost-effective and practical choice for homeowners looking to expand on a smaller scale, but for those seeking a more dramatic increase in space and value, a double-storey extension is often the superior choice.

If you’re considering ways to maximise your home’s potential, a double-storey extension offers an exciting opportunity to create a space that truly works for you—both now and in the future.

Permitted Development Rights for Double-Storey Extensions

One of the biggest advantages of home extensions is the potential to expand without the hassle of moving. But before starting a project, it’s crucial to understand whether you need planning permission or if your extension falls under permitted development (PD) rights—which allow certain changes without a formal application.

When it comes to double-storey extensions, permitted development rights are quite limited, and most projects will require planning approval. However, if your extension meets specific criteria, you might be able to proceed without full planning permission.

Key Permitted Development Criteria for Double-Storey Extensions:

• Depth Restrictions: The extension can extend a maximum of 3 metres from the rear of the original house, on both floors. If your home already has a ground-floor extension exceeding this depth, building upwards won’t qualify under PD rights.
• Height Restrictions: The extension must not be taller than the existing eaves, and the roofline cannot exceed the height of the original house.
• Boundary Considerations: If the extension is built within two metres of the boundary, the eaves height must not exceed three metres.
• Design Limitations: Certain features are not permitted, including balconies, verandas, raised platforms, chimneys, or any alteration to the existing roof structure.
• Material Consistency: The new extension must be built using materials that match the existing house to maintain a cohesive appearance.
• Roof Pitch Requirements: The roof pitch of the extension must be similar to that of the original house.
• Proximity to Boundaries: The extension must be at least seven metres from the rear boundary and should not extend beyond the original rear wall of the house by more than three metres.
• Side Windows: If adding a side-facing window on the first floor, it must be obscure-glazed and non-opening, unless positioned at least 1.7 metres above floor level.

Where Permitted Development Rights Do Not Apply?

It’s important to note that double-storey extensions do not qualify for permitted development in certain cases, including:

• If the extension is to the side of your home rather than the rear.
• If your property is in a protected area, such as a conservation area, national park, or an Area of Outstanding Natural Beauty (AONB).

Since most double-storey extensions require planning permission, it’s always best to check with your local planning authority or consult a professional before moving forward. Proper planning ensures a smoother approval process and helps you avoid unnecessary delays.

Planning Permission for a Two-Storey Extension

If your proposed two-storey extension doesn’t qualify under permitted development rights, you’ll need to secure householder planning permission before you can begin construction.

What does this mean for your project?

• Extended Timeline: Securing planning permission may extend your project’s start date.
• Design Scrutiny: Your extension’s design must align with local council guidelines and complement the character of the surrounding area.
• Policy Alignment: Planning officers will assess whether your design adheres to regional policies and planning principles.

To ensure your proposal meets these requirements, your architects and planning consultants will collaborate to create a design that reflects your vision while complying with all necessary regulations. They will prepare detailed planning drawings and a comprehensive Design and Access Statement to support your application.

For a thorough understanding of the planning application process, refer to our detailed blog post here.

Two-Storey Extensions and Building Regulations

No matter the size of your two-storey extension, obtaining building regulations approval is a mandatory step in the process. These regulations ensure that your extension is structurally sound, energy-efficient, and meets safety standards.

There are two ways to apply for building regulations approval:

1. Full Plans Application – This involves submitting detailed drawings and structural calculations for approval before construction begins. It provides greater certainty and allows potential issues to be addressed early.
2. Building Notice – A more straightforward approach, this allows work to start without prior approval of detailed plans. However, inspections will be carried out during the build, and any non-compliance must be rectified, which could lead to delays or extra costs.

Selecting the Ideal Exterior for Your Double-Storey Extension

A double-storey extension presents an exciting opportunity to explore with exterior materials. While some homeowners choose to match the existing structure for a seamless look, there’s no rule that says you have to. In fact, incorporating contrasting materials can create a striking design that enhances your home’s character.

Sustainable and cost-effective options are worth considering. Bamboo, for example, offers a stylish and eco-friendly choice for exterior walls, especially when paired with a green roof. Reclaimed timber or brick not only add warmth and charm but also promote sustainability. Unlike newly sourced wood, reclaimed timber has already weathered, ensuring a more predictable aging process.

For a sleek and modern finish, metal cladding is an excellent choice. Steel, aluminium, zinc, or copper provide durability, low maintenance, and a contemporary aesthetic that can make your extension stand out. Whether you prefer a bold contrast or a harmonious blend, the right materials can transform your extension into a stunning and lasting addition to your home.

Double Storey Extension to a  Detached House

When it comes to a large two-storey extension, detached houses offer the greatest flexibility. With no shared walls, there’s less concern about disturbing neighbours, making it easier to plan and execute your extension. Whether you opt for a rear or side extension, the possibilities are far more open compared to other property types.

A detached home allows for more ambitious designs, increasing the likelihood of achieving your ideal extension without major compromises. This means you have a better chance of achieving your vision, whether it’s a spacious new living area or additional bedrooms.

However, remaining open to design possibilities can be advantageous. An architect may suggest an alternative layout that enhances both aesthetics, functionality, and integrates more seamlessly with the existing structure.

Double-Storey Extension to a Semi-Detached House

For semi-detached houses, there are still several options for adding a two-storey extension, including a double-storey rear extension or a side extension on the part of your property that is not attached to the neighbouring house.

However, it’s important to note that a side extension may be more intrusive to your neighbours compared to a detached property, particularly in terms of light loss or overshadowing.

If you’re thinking about a side extension, it’s worth considering how it might impact the symmetry of the semi-detached pair. A stroll down your street can help you see if there are similar extensions nearby and whether they received planning approval in recent years.

If your neighbour has already extended to the side, your extension could help maintain the overall appearance of the property. However, there may be concerns from the council about the potential transformation of semi-detached houses into a terrace. For this reason, side extensions can be more challenging to gain planning permission for compared to rear extensions. That said, we frequently secure planning approvals for even the most complex house extensions.

 Double-Storey Extension to a Listed Building

If you’re considering adding a double-storey extension to a listed building, the first step is to apply for listed building consent.

It’s important to note that adding a two-storey extension to a listed property is a complex process. You’ll need to carefully study the original design of your house, research the materials and techniques used, and ensure that any new additions are in harmony with the existing structure. While contrast can sometimes work, it presents its own architectural challenges. Achieving the right balance is possible, but it will require more effort.

For the best results, always work with experienced listed building architects, planning consultants, and contractors to protect the historical integrity of your building.

 Double-Storey Extension in a Conservation Area

Just like with listed buildings, you’ll need to secure planning permission before adding a double-storey extension in a conservation area.

Conservation areas were established in the 1960s to protect and preserve the architectural and historical character of specific areas, as outlined in Section 69 of The 1990 Planning (Listed Buildings and Conservation Areas) Act.

The advantage of being in a conservation area is that the rules are generally clearer, particularly for a side extension. A double-storey side extension must align with the established character of the area if your home is considered a positive contribution to its aesthetic. However, you’ll have more flexibility with a two-storey rear extension, especially if it isn’t visible from the street. That said, your choices will still be more restricted than in other locations.

When hiring an architect, ensure they have experience working in conservation areas and understand the specific requirements of your area.

How Long Does a Two-Storey Extension Take?

The general timeframe for completing a two-storey extension is typically between 9 to 11 months, from the initial stages to enjoying your new kitchen or bedroom. However, it’s helpful to break the process down into three key stages:

• Developed Design & Planning: 3 months
• Technical Design & Building Regulations: 2 months
• Contract Administration & Construction: 5 months

That said, construction projects often come with unexpected delays, so these timeframes should be considered as a general guide rather than fixed milestones.

To allow for any potential setbacks, it’s a good idea to anticipate a completion date that’s at least a year from the project’s start.

How UDEX Can Help You

At UDEX, we specialize in transforming spaces to match your vision and meet your needs. We are a London-based RIBA chartered architecture and planning practice with an office in Wembley. With extensive experience in residential projects, including extensions, conversions, and new build homes, our approach combines innovative design with expert planning to create spaces that homeowners love and that developers and landowners can rely on.

If you’re considering a double-storey extension or any other type of house expansion, we’re here to help. Don’t hesitate to get in touch with us today!