Double Storey Extensions


A two-story addition can be an excellent option to increase the living area of your home. It can also be a low-cost approach to boost the value of your property.

However, before designing an extension, you should know the following. In this blog post, we will walk you through the process of planning and building a double-story expansion and give you some pointers on where to start.

What exactly constitutes a two-story extension?

A Double-story extension is typically built in the rear, side, or even front of a property to provide additional living space to your current home. If you currently have a one-story extension, a two-story extension will double the height and allow you to add an extra floor to your home.

Do you need planning clearance to build a two-story extension?

Planning clearance is required if your two-story extension is closer than 7 meters to the rear boundary. If you intend to design/plan your build with materials other than those used on the current property, authorization must be obtained and accepted before beginning.

How long does it take to construct a two-story extension?

At Ashby Facilities, we warn our clients that this extension can take anywhere from 20 to 24 weeks from design to completion. Delays may occur based on design, construction complexity, and planning approval. You must also account for any unforeseen challenges or delays that may cause your timetable to be extended.

How deep are the foundations for a two-story extension?

To lay the proper groundwork for a two-story extension, we propose a width of 600mm (up from 400mm for a single-story extension) and a depth of at least 200mm. Suppose you want to upgrade from a single to a double-story expansion. In that case, we recommend consulting a structural engineer who can inspect the foundations to verify you have the proper depth to construct upwards.

How much does a two-story addition cost?

Expect to receive quotations for a two-story expansion ranging from £60 to £80k, depending on your desired design and finish. Depending on the size of the expansion, you should expect to pay between £2 and £3k per square meter. For properties in London, this may be on the low side; therefore, expect to pay more than in other parts of the country.

Can I construct a two-story expansion over a water drain or sewer pipe?

Actually, over a repair hole or a sewer access point. Part of the process may entail moving the access point. Building Regulation H4 states that when building within 3m of a public sewer or 1m of a public lateral drain, water authority consent is required before work can begin. 

This must be factored into your design and the associated costs incurred. Remember that submitting a planning application can cause additional delays in your development.

What constitutes a permissible two-story extension?

The extension’s design and look must be consistent with the original house and adjacent properties. It must be no more than half the width of the existing residence and no higher than the current property’s eaves. The maximum length allowed is 3m for terraced or semi-detached houses and 4m for detached homes. Balconies and terraces will require planning approval.

Options for double-storey roof extensions

Double-story roof alternatives may be subject to planning permission. The roof pitch must be identical to the existing property.  A flat roof can be installed if the rooftop cannot be seen from the ground. When working with your builder on design and planning, we recommend considering both look and cost.

Can we go from a one-story to a two-story extension?

You can do it. As previously stated, you must consult with your contractor and a structural engineer to ensure that your current expansion has the foundations for a two-story addition.  If you’re constructing a single story with the possibility of expanding to a double, you should include the foundation parts in your plans.

Do I need to hire an architect for an extension?

Although it is not against the law not to have one, hiring an architect to benefit from their knowledge and experience is always preferable.

Ashby Facilities has in-house designers who can assist you with your project so that you do not have to outsource to other companies, which can save you money.

We hope you found this post helpful and instructive. If you would like to contact us for a free, no-obligation quote on your property idea, we can help make it happen! Call our friendly team today 07443234283 or fill out the contact form to discuss how we can make your dreams a reality.

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