If your family needs more space but you don’t want the hassle of moving, a two storey extension is the perfect project to consider. Offering versatility and adding value to your home, a double storey extension tends to cost less per square metre than a single storey extension. Let’s dive into the full cost of a two storey extension so you can weigh up whether it’s worth it.

How much does a two storey extension cost?

How much a two storey extension costs depends on a variety of factors, including the type of extension you choose, what you will use the extended space for and where you live, but expect an average 60m² double storey extension to cost at least £90,000.

This cost includes foundations, the walls and roof, general plumbing and electrics. You’ll need to add more if you want to add a kitchen or bathroom to your extension, plus factor in decorating costs. Here’s a breakdown of the costs:

At HaMuch, we use our database of tradespeople’s hourly rates to give you an estimate of how much home improvement projects may cost in your area. Take a look at the table further down the page to see the average cost of a two storey extension in your area.

Factors affecting the cost of a double storey extension

While we can give you an average cost of a two storey extension, there are many factors that can affect the cost.

• Size: As you’d expect, the size of your extension will affect the cost. However, you may notice a small reduction in the cost per square metre for very large extensions.

• Designer: If you ask an architect or interior designer to draw up the plans for your extension or work on the design of the space, it will cost more than if you have a builder draw up plans.

• Complexity: The more complex the design is, the more it will cost. Features like large skylights, bi-fold doors and alcoves will drive the cost up.

• Quality: A higher-quality build will obviously cost more, but you may find that you have fewer problems or have to do maintenance less often.

• Project management: If you want a builder or a project manager to manage the build of your extension, it will cost more than if you manage it yourself.

• Your location: Where you live affects the cost of building a two storey extension. Builds in London and the south east generally cost more than those in the north and more rural areas.

To get an accurate price, post a job on HaMuch and we’ll put you in touch with extension builders in your area.

How much value does a two storey extension add to a property?

How much value a two storey extension adds to a property depends on a range of things, including how well it fits in with the rest of your home, how big it is and what the space is used for. Estimates vary, but it’s said that a double storey extension adds between 12-20% to the value of your home.

The best way to maximise the profit on your home is to use an architect to draw up the plans and use an interior designer to help you make the most of your new space. While you’ll pay more for these experts, it increases your chances of finding a buyer that’s willing to pay a good price for your home when you come to sell it.

Types of two storey extension

There are a few different types of two storey extension, and which one you choose will depend on the space you have around your property and what you want to use your new extension for:

Side return

As the name suggests, a double storey side return extension is built at the side of your home. It’s a great option if you have a detached or semi-detached property and you don’t want to reduce your garden space, but you may be limited as to how far you can extend due to your property’s or your neighbour’s property boundary.

Rear

A rear extension is ideal if you’d like a larger two storey extension and you’re happy to eat into your garden space. You can usually extend further with a rear extension than a side return.

Wrap around

A wrap around two storey extension combines a side return and a rear extension to wrap around your property. It’s ideal if you want a lot of extra space for rooms like kitchens and bathrooms downstairs and bedrooms upstairs. Of course, only semi-detached and detached properties that have space at the side can have this type of extension.

Front

A front extension is the least common type of extension as you’ll change the look of the front of your property. You also have stricter building regulations to adhere to. However, if you’re short on space at the side and rear of your home, it can be a good solution.

How long does a two storey extension take to build?

Expect a two storey extension to take at least 12 weeks to build. Make sure you plan for delays, as there will always be unexpected hitches! Before and after the build, you’ll need to factor in steps such as getting planning permission or party wall agreements and finishing touches, which will add to the overall project time.

Two storey extension build timeline

Pre-build

• Planning and design: 8-12 weeks

• Party wall agreements, technical drawings and building regulations: 2-8 weeks

Build

• Weeks 1-3: Foundations

• Week 4: Structural work

• Weeks 5-6: External walls

• Week 7: Ground and first floor build

• Weeks 8-9: Roofing

• Weeks 10-11: Dry lining and plastering walls

• Week 12: Electrics and plumbing

Finishing

• Week 13: Painting and decorating

• Weeks 14-15: Flooring, windows and doors

Every extension build is different, so this is just a guide timeline. You may find that your two storey extension takes less time or takes longer than this.

Do you need planning permission for a two storey extension?

There are some circumstances where you won’t need planning permission for your two storey extension, as they’re covered under permitted development. You won’t need planning permission if:

• The extension is at the rear of the property

• The property is not in a National Park, an Area of Outstanding Natural Beauty, a conservation area or a World Heritage Site

• The extension is not more than 3 metres deep, and is at least 7 metres away from the rear boundary of your property

If you do need planning permission, your application can cost up to £400 depending on your local council. Your extension will also need to comply with building regulations, and you’ll need a certificate to show this at completion. This will cost around £950.

What is the 45-degree rule for two storey extensions?

The 45-degree rule is a planning rule that all extensions must adhere to. It stipulates that any rear, side return or front extension must be set within a 45-degree line drawn from the nearest edge of the neighbouring windows. It’s essentially to ascertain whether your new extension will block light from coming into your neighbours’ windows.

Your extension’s plans will need to adhere to this rule to ensure they’re not rejected by your local building authority. An architect or whoever is drawing up your plans will angle a 45-degree line from the bottom corner of your neighbour’s closest window to your proposed two storey extension. If your extension touches the line, then it’s likely that it will block too much light from entering your neighbour’s property.

The idea behind the rule is to ensure that people can build the new properties or extensions they want without someone else losing out in the process.

Do you need an architect for an extension?

It’s not a legal requirement to hire an architect for an extension. However, it’s a good idea to consider using one.

Since a double storey extension will affect the structure of your property, it makes sense to have an architect draw up the plans to ensure your build will be safe and legal. But if you find an experienced builder that’s happy to draw up the plans, then you may not need an architect.

If you’re concerned about the cost of an architect, you could look for an architectural designer. They’re still highly qualified individuals, but haven’t completed the relevant qualifications to call themselves an architect. They can offer significantly cheaper rates.

Of course, you can also draw up plans for your extension yourself. We’d only recommend this if you have experience in the architectural or building field, as mistakes such as submitting plans that aren’t building regulations compliant can be costly.

How to finance an extension

It’s rare that people have the cash immediately available for an extension, so many homeowners borrow the money. You should always seek financial advice before you borrow large sums of money, but here are a few ways you can finance your two storey extension if you don’t have the cash up front:

• Remortgage: You can borrow money to fund your two storey extension by remortgaging. Moving your mortgage to a new lender or a new deal can release money you can use to pay your builders.

• Credit card: If you have some money available but need to borrow a small amount, for example for painting and decorating costs, you could use a credit card. Many credit cards have 0% interest offers when you first take out the credit card, so you could borrow without paying interest.

• Take out a loan: You can take out a home improvement loan to finance your extension. You will need to pay interest on the loan when you make repayments over a term – usually around 5 years. The interest rate you get usually depends on your credit rating.

Ready to get an accurate price for your two storey extension? Post a job on HaMuch now and we’ll send it to builders in your area.

Estimates for common extension builder jobs;

Rates for other trades