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A two-storey extension can be a great way to add more living space to your home. It can also be a cost-effective way to increase the value of your property. 

But, before you start planning your extension, there are some things you need to know. In this blog post, we will outline the process of planning and building a double-storey extension and give you some tips on how to get started.

What is a two-storey extension?

A two-storey extension is usually added at the back, side or even the front of a property, adding extra living space to your existing house. If you already have a single storey extension, a two-storey extension will double the height and allow you to add an upper floor to your property.

Do you need planning permission for a 2 storey extension?

If your two-storey extension is closer than 7 metres to the rear boundary, you will need planning permission. 

If you decide to design/plan your build using different materials than the existing property, then permission would also need to be submitted and approved before proceeding. 

How long to build a 2 storey extension?

At Ashby Facilities, we advise our clients that this extension can take anywhere between 20-24 weeks from design planning to build. 

Delays may happen depending on design, the complexity of the build and planning approval. You’ll also have to factor in any unforeseen issues or delays which could add to your timescales. 

Depth of foundations for a 2 storey extension?

To give a two-storey extension the right foundations, we recommend a width of 600mm – an increase up from 400mm from a single-storey extension and a depth of at least 200mm. 

If you’re looking to upscale from a single to a double-storey extension, we recommend speaking to a structural engineer who can check the foundations to ensure you have the correct depth to build upwards. 

How much does a two-storey extension cost?

Depending on the design and type of finish you require, be prepared to receive quotes between £60-80k for a 2 storey extension. 

On average, you’ll be paying around £2-3.5k per square metre, depending on the size of the extension. 

For London based properties, this may be on the low side so expect to be paying more than other areas of the country. 

Can I build a two-storey extension over a water drain/sewer pipe?

No, not over a maintenance hole or sewer access point. The access point may need to be moved as part of the work. 

Building Regulation H4 requires that when building within 3m of a public sewer, or within 1m of a public lateral drain, you’ll need water authority approval before work begins.  

This will need to be included in your design and necessary costs attributed. Remember, this can also increase delays to your development if you need to submit a planning application.

What is a two-storey extension permitted development?

The design and appearance of the extension must be of a similar design to the original house and neighbouring properties. It must be no more than 50% width of the existing home and no higher than the eaves on the current property. 

The maximum length permitted is 3m for terraced or semi-detached or 4m for a detached house. Balconies or terraces will require planning permission.

Double-storey extension roof options

Roof options for double-storey can be dependent on planning permission. The roof pitch is required to be the same as the existing property. 

If the rooftop is not visible from the ground, you can install a flat roof. We advise you to think about appearance and costs when going through your design and planning stages with your builder.

Can we go from a single to a double-storey extension?

Yes, you can. As mentioned previously, you’ll need to check with your builder and or a structural engineer that your current extension has the right foundations to upgrade to two storeys. 

If you’re currency planning a single storey with the potential to increase to a double, you may want to include the foundation elements in your planning. 

Do I need an architect for an extension?

Although it’s not against the law to not have one, We believe it is always best to get an architect on board to utilise their knowledge and expertise. 

Ashby Facilities have in house designers that can help you with your build so you’re not outsourcing to different people, which can reduce your costs. 

We hope you’ve found this article helpful and informative. If you would like to get in touch with us for a free, no-obligation quote on your property idea, we can help make it happen! 

Give our friendly team a call today or fill out the contact form and let’s talk about how we can turn your plans into reality.

You’ve got a full-time job, family commitments and want to throw home improvements into the mix with a single-storey extension project.

So here at Ashby Facilities, we’ve put together a quick guide about the type of questions we get asked when planning and building a single-storey extension.

What is a single-storey extension?

A single-storey extension is added to the back or side of a domestic property. 

It can be a maximum of four metres in height and up to six/seven metres from the rear wall. Be aware that if the build is within two metres of a boundary wall, it can be no bigger than three metres in height.

Typically used for:

• Extend your kitchen
• Downstairs bathroom
• Downstairs living space
• Extra storage space
• study/workplace

There are other types of extensions to consider too. These can be loft conversions, side extensions or double-storey extensions.

Do I need planning permission for my extension?

As a rule of thumb, if the extension is closer than seven metres to the rear boundary, permission would be required. A semi-detached house must not extend more than three metres from the back wall and four metres for a detached. 

You also need to consider the materials used in your extension. According to research from Burnetts Law Firm, the “materials used for the extension should be similar in appearance to the existing house. If you wanted to use different materials, then you would need to get planning permission for your extension.”

Building regulations for single-storey extension

Depending on the type and size of home extension, planning permission may not be required. You may need building regulations (Mike to confirm which ones if possible). If you would like to understand more, the UK government has a detailed, technical guide on permitted development rights for householders. 

How long does a single-storey extension take to build?

If permission and regulations are approved, expect a single-storey build to take twelve to sixteen weeks. Be aware of unforeseen delays such as:

• Building regulations approval delays
• Planning and other permissions, 
• Neighbours permissions – always discuss your plans with your neighbours
• Sourcing materials – delays may not come from your builder but with Brexit, oil and gas prices increasing – lead times could increase
• Sourcing good builders and labourers – delays may come from their projects over-running

How much does a single-storey extension cost

For a standard 15m2 single-storey extension, be prepared to pay in the region of £17-20,000. The bigger the extension size – the greater the cost. However, on average, the price per square metre will reduce. 

The cost of your build depends on:

• Cost for materials 
• Material lead times
• Labour
• Build lead time

Additionally, always include a budget for any unforeseen, additional costs. 

How much will a single-storey extension add to my property value?

According to uSwitch – this type of extension could add up to 23% depending on the type of property you own (semi-detached, detached, bungalow etc.

Single Storey extension roof options

Working with your builder, you can add different roof design elements to your house extension. Factors such as natural light brighten up the area with a skylight. You may not have the budget and decide a flat roof gives you the option to later extend to a double-storey. Here are some of the advantages and disadvantages of both options.

Benefits of a pitched roof single-storey extension

• Materials more water-resistant
• It doesn’t gather puddles of water and can drain off a roof = fewer leaks
• Longer construction time compared to flat roof
• Uses more materials 
• More expensive
• It can be harder to install

Benefits of flat roof extensions

• Use fewer materials
• Quicker construction time
• Can gather puddles of water and not as weather-resistant as a pitched roof
• More leaks
• Great way to save money 

How deep should foundations be for my single storey extension?

For a 3 metre single-storey extension, you need a minimum depth of 8 inches. If you are extending by four metres, we recommend ten inches to counter any cold weather or frost in the ground. If the soil or chalk is soft at these depths, you may need to go a little deeper until the ground is tougher to get through.

Pro tip – When you build an extension, there are several different types of foundations. From experience, we recommend using reinforced concrete strips.

Can I convert a single storey to a double storey?

Adding a rear extension is a great way to transform your original house, adding space and value to your property. Converting to a double will give you even more room. As a ballpark figure, expect to at least double or triple your investment from your single-storey extension.