When a single-storey extension is not enough, a double-storey extension becomes the conversation. More bedrooms. More bathrooms. A larger kitchen downstairs and an actual usable space above it. For many London families, it is the most practical way to get everything they need from one project without moving house.
But double-storey extensions are not simple. They involve more planning, more structural complexity, and more design decisions than a standard rear extension. Getting all of that right requires a team that knows what they are doing and has done it many times before across London’s varied housing stock.
Extension Architecture is one of London’s most established providers of double-storey extension services. They have delivered projects across the capital for years, working with homeowners from the first conversation through to the final handover.
What a Double Storey Extension Actually Involves
A double-storey extension adds new space on both the ground floor and first floor simultaneously. Rather than building one level and leaving the space above unused, you are maximising the full height of the build from the start.
The ground floor typically gains a larger kitchen, open-plan dining area, utility room, or additional reception space. The first floor gains one or more bedrooms, a bathroom, or an en suite. Sometimes a home office. The exact layout depends on what the family needs and what the property can physically accommodate.
What makes double-storey extensions particularly attractive from a financial perspective is the cost per square metre. Building two floors at once is significantly more efficient than completing a single-storey extension now and returning to do the first floor later. You pay for one set of foundations, one build programme, one planning application, and one set of professional fees. The total cost is higher than a single-storey project, but the space gained per pound spent is considerably better.
Planning Permission for Double Storey Extensions in London
This is the area where most homeowners need the most guidance. Unlike a modest rear extension that often falls within permitted development rights, double-storey extensions almost always require a full planning application.
That means submitting detailed drawings to the local planning authority, waiting for a decision, and designing the project in a way that satisfies the council’s requirements. In London, where conservation areas are widespread, and each borough has its own design guidance, this process needs to be handled carefully.
Here is a general overview of what the planning process looks like for a double-storey project:
| Stage | Typical Timeline | What Happens |
| Pre-application advice | 2 to 4 weeks | Optional but often useful in complex boroughs |
| Design and drawing preparation | 4 to 8 weeks | Architectural drawings, planning documents |
| Submission and validation | 1 to 2 weeks | Council confirms application is complete |
| Consultation period | 8 weeks (householder) | Neighbours notified, council assesses |
| Decision issued | Week 8 to 13 | Approved, refused, or extended |
| Conditions discharge | 2 to 4 weeks | Any pre-start conditions cleared |
A realistic timeline from starting the design to having planning permission in hand is four to six months for a straightforward London double-storey project. Complex sites, conservation areas, or sensitive neighbourhoods can add to that.
Extension Architecture manages the entire planning process. They know how different London boroughs approach double-storey applications, what design details tend to raise objections, and how to put together an application that gives you the best possible chance of a smooth approval.
Design: Making a Double Storey Extension Look Right
The design challenge with a double-storey extension is different from a single-storey project. At two floors, the new addition is more visible. It has more visual weight. And it needs to sit comfortably with the existing house rather than looking like something attached from a different building altogether.
Good double-storey extension design thinks about proportions. The relationship between window sizes on the new and existing parts of the house. The roofline and how the extension connects to it. The materials on the exterior and whether they match or deliberately contrast in a considered way.
First-floor window placement also matters from a planning perspective. Overlooking neighbouring gardens or properties is one of the most common reasons councils raise concerns about double-storey applications. Positioning, sizing, and glazing type all factor into addressing this properly.
A well-designed double-storey extension does not read as an addition. It reads as part of the house. That takes design experience and an understanding of what actually gets approved in London, not just what looks good on a screen.
Structural Requirements
Double-storey extensions carry more load than single-storey ones. The first floor structure, the walls supporting it, and the foundations underneath all need to be designed to handle that load properly.
Steelwork is commonly needed where new openings are created in existing walls to connect the extension to the house. The junction between the new roof and the existing structure needs careful detailing to prevent water ingress and movement over time.
Extension Architecture works with experienced structural engineers as part of its integrated service. The architectural design and structural engineering work together from the start rather than being handled separately. That coordination avoids the gaps and contradictions that occur when different consultants work in isolation and only meet on site when something does not line up.
Building Regulations and Sign Off
Planning permission and building regulations are two separate things. Planning permission is about whether you can build. Building regulations are about how you build, covering structural safety, fire safety, insulation, ventilation, drainage, and more.
For a double-storey extension, building regulations sign-off is essential and non-negotiable. You will need a building control inspector to check the work at various stages throughout the build and issue a completion certificate at the end. That completion certificate matters when you come to sell the property.
Extension Architecture produces the detailed technical drawings required for building regulations submission and manages the relationship with building control throughout the project. Nothing gets missed, and nothing gets signed off before it is ready.
What Does a Double Storey Extension Cost in London?
Double Storey Extension Costs vary based on size, specification, location, and site conditions. But as a general guide, a double-storey extension in London typically ranges from £1,800 to £3,200 per square metre for the construction work alone. Professional fees, planning costs, and structural engineering add to that.
The key point is that the cost per square metre for a double-storey project is almost always lower than doing two separate single-storey projects at different times. You are buying more space more efficiently by doing it all at once.
Why Choose Extension Architecture
Extension Architecture has an established track record of delivering double-storey extensions across London. Their team covers architecture, planning, structural engineering, and project management under one roof. That means a homeowner is dealing with one team, one process, and one accountable point of contact throughout.
They know the planning requirements across different London boroughs. They design projects that get approved and that work properly as homes. And they manage the process in a way that keeps homeowners informed without overwhelming them with detail.
For a project as significant as a double-storey extension, working with a team that has done this many times before in London is not a luxury. It is the sensible approach.
Get in touch with Extension Architecture to start the conversation about your double-storey extension project.