A double storey extension adds serious space. Two additional floors of living area. For London homeowners dealing with limited square footage, this is tempting. But before you get excited about all that extra space, understand the costs. Double storey extensions aren't cheap.
Understanding what actually drives the price of double storey extensions in London, or learning how costs break down between different phases of the project, helps you make realistic decisions about whether this investment makes sense for you. If you want to know more about how Extension Architecture manages costs on these complex projects, or understand what factors most affect your final bill, you'll see why getting accurate pricing upfront matters. The cost isn't random. It's driven by specific factors. Understanding those factors helps you understand the price.
A single storey extension adds one level of space. A double storey adds two. But the cost doesn't just double. It actually increases more than that.
Why? Because structural requirements multiply. You're not just extending the footprint. You're building structure that supports two additional floors of weight. That's significant structural work.
Foundation requirements get more demanding. You need footings that can handle the extra load. Sometimes you can extend existing foundations. Sometimes you need new footings that go deeper.
Roof work becomes more complex. You're not just extending a simple roof. You might be completely restructuring the roof to support the new stories.
Connection points between old structure and new structure require careful engineering. These connections need to be solid. They need to distribute load properly.
All of this adds cost. And complexity.
Before a double storey extension even starts construction, you need structural engineering. This isn't optional. This is mandatory.
An engineer assesses your existing structure. They determine what can support additional weight. They design how to connect new structure to old structure. They specify what materials and methods are required.
This engineering work costs money. Usually 1000 to 3000 depending on complexity. For a complex property or difficult connection, it could cost more.
You're not paying for something optional. You're paying for professional assessment that makes the project actually work and safe.
Building Control requires this engineering. Without it, your project fails inspection.
What's under your house matters. Solid ground. Clay. Sand. Rock. Different ground conditions require different approaches.
If your property has good foundation conditions, you might extend the existing foundation. Cheaper option. But you need a surveyor to assess this.
If ground conditions are poor, you might need deeper footings. Or piling. Or other solutions. These cost significantly more.
A double storey extension might cost 5000 to 15000 just for foundation work depending on ground conditions. Sometimes more for difficult situations.
This is why architects london get a ground assessment done early. It affects the whole budget.
Most double storey extensions require structural steel. Steel beams to span the distance. Steel columns to carry load. These are engineered specifically for your project.
Steel costs vary based on quantity and complexity. But expect 5000 to 20000 for structural steel depending on the size of your extension.
Installation costs extra. You need skilled welders and installers. You need careful coordination with other trades.
The design of the steel affects everything else. Wall placement. Window placement. Interior layout. The structural steel determines what's actually possible.
Double storey almost always requires Planning Permission. Single storey sometimes qualifies for Permitted Development. Double storey doesn't.
Planning applications cost time and money. You need detailed drawings. You might need specialist reports. You need to submit and then wait for council decision.
Council review takes 8 to 12 weeks typically. Sometimes longer.
Some projects face objections. Neighbors object. Planning committee has concerns. You need to revise and resubmit. That adds time and cost.
A design and build company experienced with planning knows how to present projects to maximize approval odds. But planning approval itself isn't guaranteed and the process costs months.
Building Regulations inspections happen at multiple stages. Foundation stage. Structural stage. Electrical stage. Plumbing stage. Final completion.
Each inspection requires the contractor to be ready and the inspector to have availability. Coordinate these wrong and inspections get delayed. Delays extend the timeline.
Building Control approvals are mandatory. You can't skip them. They cost time but the safety assurance is worth it.
Double storey means twice as much brick. Twice as much roofing. Twice as much structural material. Twice as much flooring and wall materials.
Material costs have gone up. Brick prices increased. Timber prices increased. Steel prices fluctuate.
For a 20 square meter double storey extension, you're looking at significant material quantities. Expect 15000 to 40000 in materials depending on finishes and choices.
Cheaper materials cost less but wear faster and might not match your existing house well. Better quality materials cost more but last longer and integrate better.
Double storey requires skilled tradespeople. Structural work requires experienced installers. You can't just use anyone.
Labor costs for structural and specialist work run higher than standard building work. You're paying for expertise.
A double storey extension probably takes 16 to 24 weeks of construction depending on size and complexity. That's a lot of labor hours.
Expect labor costs to run 20000 to 50000 depending on scope and complexity. Sometimes more for very complex projects.
Your existing roof might need to come off. The new roof might be significantly more complex than what existed before.
Removing an existing roof creates waste disposal costs. Installing new roof structure costs money. New roofing materials cost money. Flashing and connections cost money. Gutters and drainage need attention.
Roof work alone on a double storey extension can run 8000 to 20000 depending on complexity and materials chosen.
Double storey probably means running new electrical circuits. New plumbing if adding bathrooms or kitchens. New heating if extending the heating system.
This work needs to meet current Building Regulations. Old systems might not meet standards anymore. You need to upgrade.
Electrical and plumbing specialist work runs 5000 to 15000 depending on extent. Could be more if you're adding significant new systems.
Once structure is complete, you need to finish the interior. Drywall. Painting. Flooring. Doors. All the finishes.
Finish quality affects cost dramatically. Budget finishes cost less. Premium finishes cost significantly more.
Expect 10000 to 30000 for interior finishing depending on quality level and size.
Every project has surprises. Hidden structural problems. Worse ground conditions than expected. Asbestos or other hazardous materials in old buildings. Utilities in unexpected places.
Budget 10 percent contingency. For a 100000 project, that's 10000 set aside for unexpected costs.
Experienced contractors build this in. Inexperienced ones don't and then run over budget.
A small double storey extension (20 to 30 square meters) probably costs 80000 to 150000 total.
A medium double storey extension (30 to 50 square meters) probably costs 150000 to 250000 total.
A large double storey extension (50+ square meters) might cost 250000 to 400000 or more.
These are rough ranges. Your specific costs depend on your specific property, your specific design, your specific location, and your specific choices.
Extension Architecture has done enough double storey extensions to estimate costs accurately. They know what affects pricing. They don't surprise clients with bills that don't match quotes.
They break costs down clearly. They explain what drives each component. They help you understand where your money goes.
That transparency is worth paying for.
