Single-Storey vs Double-Storey Extension

When you have outgrown your home, one of the first decisions is whether to extend out on the ground floor only, or to go up as well with a double-storey extension. Both can transform a London house, but they suit different needs, budgets and plots.

This guide compares the two on the things that actually matter — cost, the space you gain, planning, build time and value — so you can see which fits your situation.

Single-storey vs double-storey at a glance

Factor	Single-storey	Double-storey
Typical cost	£40,000 – £90,000	£85,000 – £160,000
Cost per m²	Higher per room added	Lower per room added
What you gain	Bigger kitchen / living space	Living space and a bedroom
Planning	Often permitted development	More likely to need permission
Build time	10 – 16 weeks	16 – 26 weeks
Best for	Open-plan ground floor	Adding bedrooms and space

Cost: total vs value for money

A single-storey extension has the lower total price, which is why it is the more common project. But a double-storey extension is usually better value per square metre: you dig one set of foundations and build one roof, then get two floors of space between them. So while you spend more overall, each room you add tends to cost less than it would as a standalone single-storey.

Planning permission

Single-storey rear extensions frequently fall under permitted development, so no planning application is needed within the size limits. Double-storey extensions can also use permitted development in some cases, but the rules are tighter — particularly on how far they project (usually a maximum of 3m) and how close they sit to boundaries. In practice, many double-storey extensions in London, especially in conservation areas, need full planning permission. Either way, both must meet Building Regulations.

When a single-storey extension makes sense

• You mainly want a bigger, brighter ground floor — an open-plan kitchen-diner opening to the garden.
• You are happy with the number of bedrooms you have.
• You want to stay within permitted development and avoid a planning application.
• Your budget or timeline is tighter.

When a double-storey extension makes sense

• You need another bedroom or bathroom as well as more living space.
• You plan to stay long term and want to future-proof the house in one project.
• Your plot and street can take the extra height without overshadowing neighbours.
• You want the best cost per square metre and the biggest uplift in value.

Impact on value

Both add value in London, but a double-storey extension usually adds more in total because it can add a bedroom, which can move the house into a higher price bracket. A single-storey extension still gives a strong return when it creates the open-plan living space buyers want. As always, the ceiling price for your street sets the limit on how much value any project can add — building beyond it rarely pays back.

How Alstruct helps you decide

Because we design and build extensions in-house, we can model both options for your specific house — what each would cost, what you would gain, and the likely planning route — before you commit to a direction. One accountable team then handles the drawings, structural engineering, planning and construction, with a 10-year workmanship warranty on the finished work.

Frequently Asked Questions