Double Storey Extensions

Double storey extension foundations will typically require 0.6-1.5 meters deep, but it can also depend quite a bit on soil, trees, proximity, drainage, and nearby buildings. We can do that for you as structural engineer.

Why This Is a More Complex Question Than It Appears

Question of foundation depth for a double storey extension can seem straightforward, but in reality is site specific and one size fit all cannot be used. Here is why foundation requirements are so varied:

Soil Type and Composition

London Clay: London Clay is subject to shrink-swell. Foundation levels have to go as low as 1 – 1.5 meters to get below the ground area affected by seasonal moisture variations.

Sandy Soils: While providing better drainage, sandy soils provide unreliable bearing capacity, sometimes requiring wider rather than deeper foundations or special design considerations.

Chalk or Bedrock: Areas with bedrock close to the surface might support shallow foundations (0.8-1.0m) but often require specialized excavation equipment. Which can increase the cost for your foundation.

Made Ground: Previously established ground may have variable fill material with the need for deeper foundations to access undisturbed ground or other foundation solutions such as piling.

Tree Influence Zones

Existing Trees: Older trees are able to have a dramatic effect on foundation needs. As an example, an oak tree in close proximity to your extension, say 15 meters, may require foundations of up to 3-3.5 meters due to its large root system and high water requirement.

Tree Species: Different species affect soil in different ways. Poplars, willows, and oaks are particularly well known for reducing soil through water extraction.

NHBC Guidelines: These set out specific foundation depth requirements by tree species, mature height, distance from the building, and soil type. These calculations typically result in foundation depths far in excess of normal minimums.

Ground Conditions and Water Table

High Water Table: Foundations of properties where groundwater levels are high require careful foundation design and could require tanking, drain systems, or particular foundation systems.

Sloping Sites: Building on an incline generates additional lateral stresses to be borne by foundations and could require step foundations or the installation of retaining structures.

Flood Zones: Dwellings on flood grounds necessitate water–pressure-capable foundations and probable erosion-resistant ground structures.

Structural Loading

Wall Construction: Solid build walls pass on heavier weights compared to timber-frame building and can require wider foundation footings.

Internal Layout: Open plan layouts with tall spans can put loads where, and therefore more foundation support may be required at these locations.

Additional Storeys: While we are having double storey extensions, room for additional further development must be considered when planning the foundations.

Adjacent Structures

Proximity to Boundaries: Building near boundary properties may include individual foundation solutions to avoid undermining the adjacent buildings.

Existing Services: Underground utilities may limit excavation depth or require foundation bridging solutions.

Party Wall Considerations: Building near boundary lines are likely to require party wall arrangements and can influence foundation design.

Cost Implications of Foundation Variations

Remember that foundation details can significantly impact your project cost:

Each additional meter of foundation depth increases building costs by £5,000-£8,000

Other specialist foundation options like piling can increase foundations costs by 50-100%

Bad foundations, on the other hand, can lead to structural issues costing tens of thousands to repair

Understanding Your Planning Options

Permitted development right mean you can make changes to your house without the need to apply for planning permission. It often speeds up the process, cheaper and alot easier. The regulations for Double Storey extension or two storey extension are more stringent. It is unfortunate that most Double Storey extensions require householder planning permission, though specific exceptions exist that could save you significant time and cost.

When Permitted Development Rights Apply

Your two-storey house extension may qualify under permitted development if it precisely meets all of these criteria as specified in Schedule 2, Part 1, Class A of the Town and Country Planning (General Permitted Development) Order 2015 (as amended):

• Depth restriction: Maximum projection of 3m from the original rear wall on both ground and first floors
• Height limitations: Eaves cannot exceed the height of existing house eaves, and the ridge height must remain below the existing roof
• Boundary considerations: Must maintain a minimum 7m distance from any boundary.
• Boundary height restriction: If within 2m of any boundary, eaves height cannot exceed 3m
• Matching aesthetics: Exterior materials and roof pitch must closely match the existing property
• No additional features: Cannot include balconies, verandas, raised platforms, or alterations to the existing roof.
• Window restrictions: Any first-floor side windows must use obscure glazing and be non-opening unless positioned 1.7m+ above floor level
• Volume restrictions: The total volume of all extensions must not exceed 40 cubic meters for terraced houses, or 50 cubic meters for detached and semi-detached houses (or 10%/15% of the original house volume, whichever is greater)

If you plan on doing a side extension you will not qualify for permitted development Side extensions: Any two-storey extension to the side of your property requires planning permission regardless of size (Schedule 2, Part 1, Class A.1(h)(i)) also Protected designations: Your property is within a conservation area, AONB, national park, or World Heritage Site (Schedule 2, Part 1, Class A.2). 

It is worth checking with your local council as sometimes Permitted development rights may be removed by a previous development or used up. 

Technical Considerations Under Building Regulations

Even if planning permission isn’t required, your two-storey extension must comply with Building Regulations, including:

• Structural stability: Calculations must demonstrate adequate foundations and support
• Fire safety: Fire-resistant construction and adequate means of escape
• Energy efficiency: Meeting current Part L requirements for thermal performance

Ventilation and damp-proofing: Compliance with Parts F and C regulations

The honest truth is detached houses are often to most easiest to obtain planning permission for a detached house as it would have the least impact on neighbouring properties, you may be able to choose between side double storey extension or a rear double storey extensions.

Rear Extensions

A double-storey rear extension creates substantial additional living space while maintaining your home’s street appearance. Imagine stepping from your hallway into a spacious, light-filled kitchen-dining area that opens completely to your garden through expansive sliding doors.

Upstairs, you might create an additional bedroom with an ensuite or space to reconfigure the entire upstairs. Many homeowners incorporate dramatic double-height spaces that transform the feeling of the house, creating moments where light cascades down through both floors. These spaces often become the heart of family life, where everyone naturally gravitates.

Rear extensions offer privacy from neighboring properties while creating visual focal points that can only be appreciated from within your home and garden

Side Extensions

Side extensions are a great way of adding another living room, kids’ room, or office on the ground floor. On the first floor, they provide the benefit of an additional bedroom and, most importantly, provide more coverage to your roof. So if you plan on doing a loft conversion, this becomes particularly valuable. Usually when doing a two-storey extension, some councils do not prefer hip to gable conversions and would like you to retain the existing hip roof. A side extension will provide ample space for your staircase to work effectively. You might create a new primary entrance with greater impact – perhaps a double-height hall that makes arriving home a special experience.
On the upper floor, you might add bedroom with an ensuite Side extensions can dramatically enhance your home’s profile when viewed from the street, especially when designed with architectural features that either complement or thoughtfully contrast with the original building.Some homeowners incorporate garages or carports with living space above – practical yet beautiful additions that solve multiple needs simultaneously.

It is worth noting that side extensions cause more harm than rear extensions on a detached property as they change the complete look of the property when viewed from the outside, they can potentially block your neighbours sunlight. Often double storey side extensions will be set back at ground from level and first floor or just a first level depending on the local council design guide.

Wrap-Around Extensions

Wrap around extensions are one of the best double storey extensions you can do, however with that comes great restrictions, let talk about the benefits of this, Wrap around extensions can create different zones that still feel connected. Corner glazing can capture multiple aspects of your garden and bring light from different directions throughout the day. Wrap around extensions often attract alot of neighbouring complaints as it can block their light to the rear and side, these applications must be handled correctly.

Of course you can do a double-storey extension to your semi-detached property. This popular home improvement offers substantial additional space without the hassle and expense of moving house.

Semi-detached homeowners typically have two main options: a double-storey rear extension or a double-storey side extension on the side which is not attached. As with detached houses, rear double storey extension generally face few planning issues, especially when designed to minimize light obstruction to neighboring properties. These extensions are great for extending the kitchen, create open plan living areas, and adding additional bedroom or bathroom at first floor.

Side extensions as mentioned previously require a lot more thought and careful consideration. Most council have a local design guide for residential extensions such as side extensions which are double storey. Local council often like to maintain the architectural symmetry of semi-detached pairs and may have concerns about transforming the visual character into something resembling a terrace. Before proceeding you should look online at your local council to see approvals and refusal so you can effectively design your double storey extension.

With thoughtful planning and professional guidance, a double-storey extension can successfully transform your semi-detached house into a more spacious, functional home that meets your family’s evolving needs.

Yes, you absolutely need building regulations drawings for your double storey extension. These aren’t just optional documents – they’re a legal requirement for any significant home extension project in the UK.
Building regulations drawings are detailed constructions plan ( some designer only provide typical sections) others will provide details sections with callouts on specific elements of the build. Building regulations drawings are not planning drawings, they go far beyond this, planning drawings are just sketches
For a double storey extension, these drawings become even more crucial because of the increased structural complexity and safety considerations. Your drawings will need to demonstrate how the added weight and height will be properly supported, how fire escape routes are maintained, and how the extension ties into your existing home’s structure.
These are the document building control, or approved inspectors will look at and write back to you telling you whether you have complied and which section is still outstanding. Your builder and quantity surveyor will rely on these, this is why it is so important that these documents are correct and accurately represents what you plan to build.
The process typically involves working with an chartered architectural technologist who take your planning drawings, re survey the property to make sure any issues are ironed out and produce technical drawings, looking at all the approved documents. For a double storey extension, you’ll also need input from a structural engineer to calculate load-bearing requirements.
Working without proper building regulations drawings is risky – you may face enforcement action as things which are in planning may not simply work as they were made to large or too small. be required to remove or modify work that doesn’t comply, and encounter serious difficulties when eventually selling your home. More importantly, these regulations exist to keep you and your family safe in your newly extended home.
We recommend getting these drawings prepared early in your project planning phase, as they may influence design decisions and will certainly impact your budget and timeline considerations.