Three meters is a magic number for home extensions. It’s deep enough to add real space. It’s shallow enough to qualify for Permitted Development in many situations. Understanding what you can actually build with a 3 metre house extension, or learning what costs to expect and what planning rules apply, helps you understand whether this size works for your property.
If you want to know more about how a 3m extension works with kitchen redesigns, or understand the design possibilities within this specific constraint, you’ll see why this size is popular with London homeowners. Three meters isn’t huge. But it’s enough to solve real space problems if designed well.
What a 3-Metre Extension Can Include (Kitchen, Rear & Side Options)
A 3-metre extension adds meaningful space. The question is what fits in those 3 metres.
For a rear extension, 3 metres takes you from your back wall about 10 feet out. That’s enough for a proper kitchen addition. Enough for a dining area. Enough for a lounge extension.
For a side extension, 3 metres along the side creates a new room. A bedroom. An office. A utility room.
The width matters too. A 3m deep by 4m wide extension gives you about 12 square meters. That’s real space.
But 3m is also restrictive. It’s the maximum depth for Permitted Development. Go 3.1 metres and you need Planning Permission. That’s why 3m is the standard measurement.
Design within 3m requires thinking strategically. You’re maximizing every centimeter. Layouts need to work. Doors and windows need placement. Functionality needs to fit.
But three meters done well transforms how your property works. It’s not enormous. It’s just enough to solve the problem.
3m Kitchen Extension Ideas for More Space & Light
Kitchens are the most common reason for 3m extensions. Original kitchens in older homes are cramped. A 3m extension doubles the space.
The simple approach – Extend back 3m and create a kitchen-dining area. New kitchen units. Island maybe. Dining table. Two separate zones but one open space.
The connected approach – Extend 3m and knock through to the existing living room. Create one large kitchen-dining-living space. Opens up the whole floor.
The workflow approach – Extend 3m, maintain some distance. New kitchen in the extension. The current kitchen is converted into a prep area. Multiple work zones.
It is important to have lighting in a 3m extension. Garden opening bifold doors. Rooflights are the lights that take away the natural light. Windows were arranged to have a view and sunlight.
The average cost of a 3m extension of the kitchen is 25000 to 45000 in London, depending on the finishes and complexity. Outside London, 20000 to 35000.
The majority of 3m extensions to the kitchen can be classified as a Permitted Development, provided that you are not located in a Conservation Area and comply with the particular restrictions regarding the depth and garden cover, among other aspects.
Planning Permission Rules for a 3-Metre Rear Extension
It is here that 3m comes into the picture. The maximum depth of Permitted Development is three metres. This is the critical measure.
Your extension can be up to 3m deep or lower, so long as it complies with other requirements of Permitted Development, then you may not require Planning Permission.
But conditions apply:
The extension should not be more than half of the area of your garden. Therefore, 3m on a small-sized property may be beyond this.
The eaves height must not be more than the current house’s eaves height. The roof ridges can not go beyond your current roof ridges.
Other rules apply in the event that you are in a Conservation Area. Planning Permission may even be required at 3m.
Planning Permission is nearly mandatory in case of a listed building.
You have to have some restrictions in specific zones.
This is the reason why assessment is critical. The same 3m extension can be deemed Permitted Development to take place in one property and Planning Permission on another property only over the street.
Extension Architecture is an evaluation of a particular property. They are the ones that inform you that your 3m extension qualifies or that Planning Permission is required.
| 3m Extension Type | Typical Depth | Typical Width | Square Meters | Permitted Dev Possible | Typical Cost (London) | Typical Timeline |
| Kitchen Extension | 3m | 4-5m | 12-15m² | Yes, usually | £25-45k | 3-4 months |
| Rear Living Room | 3m | 4-5m | 12-15m² | Yes, usually | £25-45k | 3-4 months |
| Side Extension | 3m | 3-4m | 9-12m² | Yes, usually | £20-35k | 3-4 months |
| Dining Room | 3m | 4-5m | 12-15m² | Yes, usually | £25-40k | 3-4 months |
| Office/Bedroom | 3m | 3-4m | 9-12m² | Yes, usually | £20-35k | 3-4 months |
| Multi-Purpose | 3m | 5m+ | 15m²+ | Maybe, check limits | £30-50k | 4-6 months if PP needed |
The Cost of a 3-Metre House Extension in 2025
A 3m extension is cheaper than extensions that are bigger, as you are not constructing so much space. However, it is also similar to the cost per square meter.
In London, the 2000 to 3000 per square meter of a single storey 3m extension should be expected. That is 24000 to 36000 on 12 square meters to be built.
Buy professional fees of about 10 to 15 percent. Add contingency 10 percent. Enhance Building Regulations and approvals.
Extend 3m in total, London is likely to cost 30000 to 45000 with finishes and complexity.
Out of London, the prices are lower. 1600 and 2500 per square meter. So 19000 to 30000 for construction. Total project probably 25000 to 35000.
The costs factor in family-friendly ground, no complications, and normal finishes.
Upgrades cost more. Premium materials. Complex design. Difficult ground. These increase the total.
What’s included in this cost? Structure. Foundation. Roof. Walls. Windows. Doors. Electrical. Plumbing. Basic finishes. Building Regulations acceptance. Site clearance.
What’s usually extra? Premium finishes. Specific material choices. Complex design features. Contingency is more than 10 percent built
Design Considerations for a Functional 3m Extension
Three metres is a constraint. But good design works within constraints.
Proportions matter. A 3m extension on a large house looks small. On a small house, it looks substantial. Design proportions that work with your existing building.
Connection matters. How does the new space connect to the existing house? Doorway location. Wall removal. Layout flow. These affect how functional the space actually is.
Light matters. A 3m extension at the back of the property can be dark. Bifold doors help. Rooflights help. Windows positioned for borrowed light help.
Storage matters. Kitchens need storage. Extensions are a good opportunity for built-in storage. Shelving. Cabinetry. Makes the space more functional.
Finishes matter. Three metres of space feels bigger with light finishes. Light colors. Reflective surfaces. Good lighting design.
Flexibility matters. Design the space to work multiple ways. A kitchen that can also be a dining room. A bedroom that can be an office. Multi-functional spaces are smarter.
Why Extension Architecture Recommends 3-Metre Extensions
Three metres is popular for good reasons. It’s deep enough to solve real problems. It’s shallow enough to often qualify for Permitted Development. Costs are reasonable. Design is manageable.
For many London homeowners, a 3m extension is the sweet spot. Bigger extensions cost more and take longer. Smaller extensions don’t solve the space problem.
A 3m extension assessed and designed by Extension Architecture typically qualifies for Permitted Development. That saves months compared to Planning Permission required projects.
The design fits the constraint. Three metres done well is better than a larger extension done poorly.
If you’re considering an extension for your London property, evaluate whether 3m solves your problem. Get a professional assessment. Understand whether Planning Permission is needed. Get detailed cost estimates.
Extension Architecture helps homeowners evaluate whether 3m extensions work for their specific properties. They design 3m extensions that maximize space. They navigate Permitted Development rules. They deliver projects that transform how people live in their homes.
For a 3m extension that works, get professional guidance specific to your property. See more