Two-storey extension by Bradley Van Der Straeten Architects. Photo by French + Tye.
Building a two-storey extension offers many advantages over a single-storey addition. It can be more cost-effective, preserve garden space, and balance your home's layout.
This guide outlines, in a straightforward way, the main planning and legal considerations if you’re thinking about building more than one storey.
Understanding planning requirements is crucial before starting your two-storey extension project. Without this knowledge, you risk delaying the project and incurring unnecessary costs.
Before you put pen to paper, check whether your project falls under permitted development (PD) rights or requires full planning permission.
Permitted Development (PD) Rights are national planning legislation that allows homeowners to build certain projects without planning permission. They set out clear guidelines on what is permissible. If your design meets PD requirements, you are legally entitled to build without applying for planning permission.
Full Planning Permission is more nuanced. Rules are often less specific than PD rights and are set by Local Planning Authorities, varying from one area to another. LPA will evaluate your plans against local policies, consult neighbours (most PD applications don’t involve consultation), and decide whether to approve or refuse your proposals.
If PD applies to your project, it’s generally the best route, offering more certainty from the outset.
Double storey extension to a detached house at the end of a cul-de-sac by Artefact Studio. Photo by Lorenzo Zandri.
Homeowner PD rights only apply to houses on land free of planning controls.
You'll need to apply for planning permission for a two-storey extension if:
Key rules for two-storey extensions include:
An Illustration showing the main permitted development sizing rules for two-storey extensions.
Meeting PD guidelines can be difficult for attached properties or homes in built up areas where boundaries are close together. As a result, most two-storey extensions you’ll find online are to semi-detached or detached homes on generous plots.
In urban areas or designated land, full planning permission may be your only option.
LPAs often require extensions to:
Note the more nuanced terminology compared to PD guidance. This approach is less prescriptive, making the planning process a little trickier to navigate.
Consult your local planning authority’s Local Plan and supplementary guidance documents for specific requirements. An experienced residential architect can streamline this process for you.
This two-storey extension in a conversation area by Sam Tisdall Architects was subject to strict planning requirements and needed to closely match the appearance of the existing building. Photo by Killian O'Sullivan.
When assessing full planning applications, LPAs use the 45-degree and 25-degree rules of thumb from the BRE to evaluate the impact on neighbouring properties’ natural light.
The 45-degree rule applies when an extension projects at right angles from a neighbouring habitable window – windows to living, dining or bedrooms, excluding bathrooms, halls, and utility spaces. If the window’s centre lies within a 45-degree angle from the extension’s outer edge (both in plan and elevation), significant light may be blocked.
The 25-degree rule applies to habitable windows opposite the extension. If the proposed development exceeds the projected angle, it may affect daylight.
Designing within these constraints simplifies the approval process.
Note that these rules don’t apply to PD applications.
An illustration of the 45-degree rule. The proposed extension must respect the 45-degree line when drawn from the centre of the closest neighbouring habitable window, both in plan and elevation. Otherwise, a sunlight and daylight assessment may be required.
An illustration of the 25-degree rule. Keeping your extension below the 25-degree line of windows opposite will improve your chances of obtaining planning permission.
If your plans don’t comply with the 45-degree and 25-degree rules, a daylight and sunlight analysis may be required.
This assessment evaluates the extension’s impact on neighbouring properties using 3D modelling software, comparing results against BRE guidance. If needed, integrate the findings into your design before submitting the application.
Rights to Light (RTL) is a legal easement distinct from planning legislation. It ensures property owners maintain rights to natural light after 20 years. Unlike daylight and sunlight assessments, these rights apply to all spaces, not just habitable rooms.
Even small extensions can infringe on RTL, potentially leading to disputes or injunctions. Importantly, securing planning permission or building in accordance to PD guidance does not override Rights to Light. An injunction could still force costly design changes or, in extreme cases, partial or complete demolition.
Appointing a specialist surveyor early in the design process can help avoid such issues. Many surveyors can conduct both daylight sunlight and Rights to Light assessments simultaneously.
Even if your plans comply with PD, apply for a Lawful Development Certificate to confirm compliance.
For full planning applications, consider seeking pre-application advice to identify potential issues early. Submit accurate drawings, including floor plans, elevations, site plans, and supporting documents like a Design and Access Statement.
LPAs typically decide within 8–13 weeks for householder applications. If refused, you can appeal or revise and resubmit based on feedback.
Two-storey extension by Emil Eve Architects. Photo by Taran Wilkhu.
Regardless of planning permission or permitted development, your extension must comply with building regulations.
If your proposals affect a shared wall or boundary, you’ll need to serve a Party Wall Notice to your neighbours.
For extensions near public sewers, obtain a build-over agreement from your water company for compliance.
Building a two-storey extension is an excellent way to enhance your home. Engaging an experienced residential architect will ensure you navigate the planning and construction process effectively.
The information provided in this article is intended for general guidance and educational purposes only. For specific expert advice on your project, consult a skilled residential architect featured on Designs in Detail.
About the Author
Aron Coates is an architect with over two decades of experience, working on projects ranging from historic building restoration to contemporary home design.
