Single Storey vs Double Storey Extension: Pros and Cons

When your home no longer fits your family, extending is often the smartest alternative to moving. It keeps you in the area you love, avoids stamp duty and estate agent fees, and lets you design the extra space exactly as you want it. But the first big decision comes early: should you build out with a single storey extension, or up and out with a double storey one?

Both options can transform a house, yet they differ in cost, planning requirements, disruption and the kind of space they create. This guide compares single storey vs double storey extensions side by side, so you can choose the right route for your property, your budget and your plans.

Single Storey Extensions Explained

A single storey extension adds one level of living space, usually at the back or side of the house. It is the most familiar type of home extension in the UK and is commonly used to create a larger kitchen-diner, a family room or a utility space. Variations include rear, side-return and wraparound designs.

Common Uses

Most homeowners choose a single storey extension to improve the ground floor. Typical projects include open-plan kitchen and living areas, a garden-facing dining space or a ground-floor bedroom and shower room for accessibility. Because it connects directly to the garden, it also improves how the house feels in summer.

Pros of a Single Storey Extension

Often Simpler Planning

Single storey extensions frequently fall within permitted development rights, which means formal planning permission may not be required. Limits on depth, height and proximity to boundaries still apply, so it is essential to check before you start. Building regulations approval is needed in almost every case.

Lower Overall Cost

Because you are building less, the total outlay is smaller. For homeowners working to a firm budget, this is often the deciding factor.

Less Structural Complexity

A single storey build does not need to carry the weight of another floor. Foundations, walls and roofs are generally more straightforward, which can mean a shorter programme and fewer surprises.

Less Disruption Upstairs

Most of the work happens outside or at the back of the house, so bedrooms and bathrooms on the upper floor are usually unaffected during construction.

Easy Connection to the Garden

Bi-fold doors, sliding glazing and roof lanterns can flood a single storey extension with light and create a seamless link to the outdoors.

Cons of a Single Storey Extension

Uses Up Garden Space

Every square metre you build on is a square metre you lose outside. On small plots, this can limit how much space you can add, and it may reduce the garden you enjoy.

Higher Cost Per Square Metre

A single storey extension typically costs more per square metre than a double storey one, because the foundations and roof are shared across less floor area. Smaller extensions are especially affected, since many fixed costs remain the same regardless of size.

Limited Extra Space

You only gain as much floor area as your plot and planning limits allow. If you need several new bedrooms, a single storey extension may not be enough.

Planning Limits Can Restrict Design

Permitted development rules cap depth and height, so very ambitious designs may need a full planning application.

Double Storey Extensions Explained

A double storey extension adds two levels at once, typically a ground-floor living space with new bedrooms or a bathroom above. It can be built at the rear, the side or both, and it is a popular way for growing families to gain a lot of space within the footprint they already own.

Common Uses

Typical projects pair a larger kitchen-diner downstairs with a new master suite or extra bedrooms upstairs. Some homeowners also use the upper floor for a home office, a nursery or a bathroom that eases the pressure on the main one.

Pros of a Double Storey Extension

Much More Space

The biggest advantage is the sheer amount of room you gain. One build can transform both floors of the house, which is difficult to match with a single storey design.

Better Value Per Square Metre

Because a double storey extension shares one set of foundations and one roof across two floors, it usually costs less per square metre than a single storey version. According to Homebuilding’s 2026 guide, a double storey project can provide roughly twice the floor space for a more modest increase in total cost, in the region of 50 to 70 per cent more.

Smaller Footprint on the Garden

By building upward as well as outward, you gain extra floor area without sacrificing as much of the garden.

Strong Potential Value Uplift

Adding bedrooms and bathrooms can significantly improve a home’s appeal and, in many areas, its market value. A local estate agent can tell you what an additional bedroom is worth in your street.

Combined Disruption

Doing both floors at once means one project, one set of scaffolding and one period of disruption, rather than two separate builds.

Cons of a Double Storey Extension

Higher Total Cost

Even if the price per square metre is lower, the overall budget is larger. A double storey build may simply be out of reach for homeowners with tighter funds.

More Likely to Need Planning Permission

Double storey extensions generally require a householder planning application, as they have a greater impact on neighbours and the street. Considerations include light, privacy, overshadowing and how the design matches the existing house.

More Complex Construction

Carrying a second floor demands stronger foundations, careful structural design and more detailed coordination between trades. It is vital to appoint experienced professionals.

Greater Disruption to the Existing Home

Because the new extension connects to the upper floor, there will be more internal work, such as opening up walls, connecting rooms and altering the roof. Some rooms may be out of use for a period.

Neighbour Concerns

A taller structure is more noticeable. Early conversations with neighbours and, where necessary, a party wall agreement can prevent disputes later.

Single Storey vs Double Storey Extension: Side-by-Side Comparison

Cost

Homebuilding’s 2026 estimates, excluding VAT, put single storey extensions at roughly £2,200 to £3,300 per square metre, while double storey extensions sit at roughly £2,000 to £3,200. Costs rise in London and the South East, and they vary according to specification, ground conditions and how the project is managed. Treat all figures as guides and get itemised local quotes.

Planning and Regulations

Single storey projects often qualify for permitted development, while double storey projects almost always need planning permission. Both need building regulations approval, covering structure, fire safety, insulation, drainage and electrics. If you share a boundary wall, you may also need to serve party wall notices.

Timescales

Smaller single storey projects are quicker. At Sussex County Construction, most extensions take around 8 to 16 weeks, with the programme depending on size and complexity. A double storey extension will usually sit towards the longer end of that range or beyond it, plus time for planning.

Added Value and Use

A single storey extension mainly enhances daily living and the feel of the ground floor. A double storey extension adds both living space and bedrooms, which tends to have a bigger effect on how many people your home can comfortably house and how buyers see it.

Light and Design

Single storey designs excel at bringing in daylight through glazing and roof lanterns. Double storey designs allow more architectural variety but need careful planning so that neither floor loses light.

Which Is Right for You?

Choose a Single Storey Extension If:

• Your main goal is a bigger kitchen or living space.
• You have a generous garden and want to keep costs down.
• You would like a simpler planning route.
• You want minimal disruption to the upper floor.

Choose a Double Storey Extension If:

• You need extra bedrooms or bathrooms, not just a bigger ground floor.
• Your garden is small and you want to limit the footprint.
• You want the best value per square metre and can afford a larger budget.
• You are happy to go through a full planning application.

If you are still unsure, ask a designer or builder to sketch both options. Seeing them side by side often makes the answer obvious.

Planning Your Extension Budget

Whichever route you choose, keep a contingency of at least 10 per cent for the unexpected, such as unforeseen ground conditions, drainage issues or structural surprises. Remember to include professional fees, planning and building control charges, VAT where applicable, kitchen or bathroom fittings, flooring, decorating and landscaping. Itemised written quotes help you compare like with like.

How to Choose the Right Extension Builder

The quality of the team matters as much as the design. Look for a builder with experience of the specific type of extension you want, a portfolio of finished projects and clear communication. Ask about insurance, guarantees, building regulations sign-off and who will manage planning and structural design.

Working with a company that handles the whole process, from design and approvals through to construction and handover, takes a great deal of stress out of the project. The team at Sussex County Construction specialises in house extensions, including single storey, double storey, rear, side and wraparound designs, and manages the planning paperwork on your behalf.

Design Tips for Either Type of Extension

Good design makes any extension feel like it was always part of the house. Match brickwork, roof pitch and window proportions to the original building, and think about how the new space connects to the old one. Aim for generous glazing, sensible storage and a layout that works for how you actually live. Consider future needs too, such as a growing family or a home office, so the extension stays useful for years.

Common Questions About Single and Double Storey Extensions

Can I add a second storey later?

Sometimes, but it is far easier to plan for it from the start. A single storey extension built without stronger foundations may not support another floor, so tell your designer early if you might want to build upward in future.

Will a double storey extension add more value?

Often it will, because extra bedrooms and bathrooms are highly valued by buyers. However, the uplift depends on your area, the quality of the build and the cost of the work, so ask a local estate agent before you decide.

Can I live at home during the build?

Many families do. Single storey projects tend to cause less disruption, while double storey projects involve more internal work. A good builder will protect the rest of the house and agree a clear plan with you in advance.

Do I need an architect?

Not always. Many experienced builders work with architects, designers and structural engineers as part of one coordinated service, so you receive drawings, calculations and approvals without managing each professional yourself.

Your Extension Decision Checklist

• Define why you need the space and how you will use it.
• Set a realistic budget with a 10 per cent contingency.
• Check permitted development rights and planning requirements.
• Consider neighbours, light and party wall issues.
• Get itemised written quotes from experienced builders.
• Compare single storey and double storey designs side by side.
• Confirm timescales, payment stages and guarantees.

Final Thoughts

There is no single right answer in the single storey vs double storey debate. A single storey extension is usually cheaper, quicker and simpler to approve, while a double storey extension offers far more space and better value per square metre if you can invest more. The right choice depends on your budget, your plot and how you want to live in your home.

If you are weighing up your options in West Sussex, Sussex County Construction offers free site visits and clear, honest advice. Get in touch to talk through your plans and find the extension that suits your home.