An extension is the most common and often the most transformative thing a homeowner can do. More space, better light, rooms that work the way your life actually works rather than how a 1930s or 1970s developer imagined it. Done well, an extension adds genuine value and function to a house. Done without enough planning, it adds disruption, cost and sometimes regret.
This guide covers the whole process: whether you need planning permission, how to design something worth building, what it will cost, and how to manage the build. It applies primarily to rear and side extensions on houses in England, which account for the vast majority of domestic extension projects.
Permitted Development or Planning Permission
The first question for any extension is whether it falls within permitted development (PD) rights. Permitted development allows certain types of work without a formal planning application, subject to size limits and conditions.
For single storey rear extensions on a detached house, PD allows up to 4 metres depth from the original rear wall (8 metres under the Neighbour Consultation Scheme, subject to no neighbour objections). For semi-detached and terraced properties, the standard limit is 3 metres (6 metres under the NCS). The extension must not exceed 4 metres in height.
For a double storey rear extension, PD allows up to 3 metres depth, must be at least 7 metres from the rear boundary, and must not exceed the height of the existing roof. Side extensions of more than one storey are not permitted development and always require full planning permission.
Single storey side extensions can be PD if they're no more than half the width of the original house and no higher than 4 metres.
These limits apply to the original house as built (or as it stood in 1948, whichever is earlier). Any previous extensions count against the limits. If you're in an Article 4 Direction area, a Conservation Area, or a listed building, PD rights may be removed or restricted entirely.
If your project sits comfortably within PD limits, you can apply for a Lawful Development Certificate (LDC) to confirm this in writing. This is not legally required to proceed with PD work, but it provides formal confirmation that is valuable when you come to sell.
The Neighbour Consultation Scheme is underused. If your single storey extension is between 4-8 metres deep (detached) or 3-6 metres deep (semi/terrace), you can proceed after a 42-day consultation period during which neighbours can raise objections. The council then decides whether to allow it. Most proceed without issue. It's worth doing rather than redesigning to fit the standard limits.
Design Principles That Actually Matter
The physical rules of what you can build are only one part of the design question. The harder part is designing something that will genuinely improve how your house works and looks.
Understand the existing layout first. Most problematic extension designs try to add space without thinking carefully about how it connects to what's already there. A large kitchen extension that requires you to walk through the dining room to reach it is less useful than a smaller one with better connectivity. Spend time in your house understanding how you actually move through it before you start sketching options.
Light is as important as floor area. A modestly sized extension that's well lit is a far better room than a large one that's dark. Think about roof lights, glazed doors, and the orientation of the extension relative to the sun. North-facing extensions in particular need careful thought to bring in adequate daylight.
Consider the garden relationship. Extensions that open well to outdoor space feel significantly larger than they are. Bifold or sliding doors across the rear elevation are popular for a reason: when the weather permits, the space doubles. Plan for this even if you don't install the doors immediately.
Don't underestimate the structural implications. Opening up the back of a house to create an open plan kitchen/living space requires removing walls, which requires structural support. Steel beams, padstones and new load paths need a structural engineer's input, not just a builder's opinion.
Get an architect involved early. Even for a relatively simple extension, an architect's input at the design stage is worth the fee. They will identify problems you haven't thought of, suggest options you wouldn't have considered, and produce drawings that are fit for planning and building regulations. A builder-drawn extension on a napkin is a false economy.
Making a Planning Application
If your project requires planning permission (either because it exceeds PD limits, you're in an Article 4 area, or you want a full planning consent for certainty), the process in England is:
Applications are submitted online via the Planning Portal. You'll need: site location plan (1:1250 scale), existing floor plans and elevations, proposed floor plans and elevations, and usually a design and access statement for anything beyond the most basic proposal. Your architect will produce these.
The application fee for a householder extension is currently £258 (as of 2025). The council has 8 weeks to determine a standard householder application. In practice, this is often longer due to information requests or officer workload.
If you're in a sensitive location (Conservation Area, AONB, Green Belt boundary), expect more scrutiny of design and materials. Pre-application advice from the council's planning department is worth seeking for anything complex: it costs money but can save you a refusal and resubmission.
Single vs Double Storey: The Trade-offs
The choice between a single and double storey extension depends on your needs, budget, site, and what planning will permit.
Single storey advantages: Usually cheaper per square metre than a double; simpler structurally; better light penetration into the ground floor; can often proceed under PD. Ideal if your main need is a larger kitchen, utility room, or living space.
Double storey advantages: More space for the same footprint cost (foundations, drainage, roof, connection to house); more bedrooms or bathrooms without consuming all the garden; better value per square metre once the ground floor structure is accounted for.
The rough rule: if you need both ground floor and first floor space, a double storey extension is almost always better value than two separate extensions. If you only need ground floor space, a single storey is often the right answer and is more straightforward to plan and build.
A word on building over neighbours: side return extensions, which fill in the narrow gap between the house and the boundary in terraced and semi-detached properties, are popular but proximity to the boundary and the neighbour's wall triggers both party wall notices and, in some cases, planning conditions about light. Get advice before assuming a side return is straightforward.
Extension Costs
Extension costs vary by location, specification, and market conditions. The figures below represent typical ranges for England in 2025 and assume a reasonable but not luxury specification.
| Project | Approximate cost range (2025) |
|---|---|
| Single storey rear extension, 15-20m2 | £40,000 - £65,000 |
| Single storey rear extension, 25-35m2 | £65,000 - £100,000 |
| Double storey extension, 30-50m2 total | £90,000 - £150,000 |
| Side return extension (typical terrace) | £45,000 - £75,000 |
| Wraparound extension (side + rear) | £90,000 - £160,000 |
These figures include construction, structural steelwork, glazing, and basic finishes (plaster ready for decoration). They exclude professional fees (architect, structural engineer, project management: allow 12-18% of build cost), planning fees, furniture, fitted kitchen or bathroom if included in the extension, landscaping, and VAT.
London and the south-east command a significant premium over these figures, sometimes 20-35% above. Scotland, Wales, and Northern Ireland have their own planning frameworks and different cost profiles.
A contingency of 15% is not overcautious for an extension project. Unforeseen ground conditions, unexpected structural issues when opening up walls, and changes to specification during the build are the three most common sources of cost overrun.
The Build Sequence
Understanding what happens in what order helps you track progress and know when to be on site. A typical extension follows this sequence:
Groundworks and foundations. Excavation, drainage diversions if needed, concrete foundations. Building control inspection at excavation stage before pouring. This is usually the most disruptive phase in terms of site access and noise.
Superstructure. Walls (blockwork or timber frame), structural steelwork, first floor structure (if double storey), roof structure. Building control inspections at various structural stages.
Weathertight. Roof covering, windows and external doors installed. Once the building is weathertight, internal work can proceed in parallel without dependency on weather.
First fix. Services run before walls and ceiling are lined: electrical cables, plumbing pipework, underfloor heating pipes (if specified). Boiler Upgrade Scheme and heat pump work often happens around here.
Insulation and boarding. Cavity wall insulation, roof insulation, plasterboard to walls and ceiling. Airtightness details at junctions.
Plastering. Wet plaster or skim to plasterboard. Allow 2-4 weeks drying time before decoration.
Second fix and finishing. Electrical fittings, plumbing connections, floor finishes, skirting, decoration. Building control final inspection and completion certificate.
A typical single storey extension takes 12-16 weeks from groundworks starting to practical completion. A double storey is usually 16-24 weeks. These timelines assume no major delays from weather, material supply, or subcontractor availability.