Single Storey vs Double Storey Extension: Cost, Time & Value Compared

Going single or double storey is one of the first decisions when planning an extension. The answer isn’t always obvious — a double storey costs more but gives you significantly more space per pound spent. Here’s how to work out which makes sense for your property and budget.

Cost Comparison

Single-Storey Extension: £30,000–£60,000

The range depends on size, specification, and what the room is for. A simple 3m x 5m living room extension at the lower end. A large kitchen-diner with bi-fold doors, underfloor heating, and high-spec finishes at the upper end. For Dorset, budget around £1,800–£2,500 per square metre fully finished.

Double-Storey Extension: £50,000–£90,000

Here’s the interesting part: a double storey doesn’t cost double. The foundations, groundwork, and roof are largely the same whether you build one storey or two. The extra cost is mainly the additional blockwork, floor structure, and internal fit-out of the upper floor. Typically, the second storey adds 40–60% to the cost of a single storey, while doubling the floor space. That makes the cost per square metre significantly lower.

On a like-for-like footprint, you might pay £45k for a single storey or £68k for a double — but the double gives you twice the space for only 50% more money.

Value Added

• Single-storey extension: typically adds 10–15% to property value

• Double-storey extension: typically adds 15–20% to property value

The double storey adds more value because you’re gaining an extra bedroom (or two) on the upper floor, which has a bigger impact on property valuation than extra living space alone. Just like with loft conversions, moving from 3 bedrooms to 4 can shift your property into a higher price bracket.

Planning Permission

Single-Storey Permitted Development

Under permitted development, you can build a single-storey rear extension up to:

• 6 metres from the original rear wall (semi-detached/terraced)

• 8 metres from the original rear wall (detached)

• Maximum eaves height of 3 metres

This covers a decent-sized extension without needing a planning application. However, the "prior notification" process may apply for extensions between 4–8m, where neighbours are consulted.

Double-Storey Planning Requirements

Double-storey extensions almost always need planning permission. The permitted development rules for two-storey rear extensions are much tighter:

• Maximum depth of 3 metres from the original rear wall

• Must be at least 7 metres from the rear boundary

• Eaves height must match the existing house

In practice, most double-storey extensions exceed these limits and need a full application. Budget an extra 8–10 weeks for the planning process.

Build Time

• Single storey: 12–16 weeks

• Double storey: 16–24 weeks

The double storey takes longer at every stage: deeper foundations take more time to dig and cure, there’s more blockwork, more internal finishing, and the second-floor fit-out (bathroom, bedroom, etc.) adds weeks to the programme.

When a Single Storey Makes More Sense

• You only need ground-floor space. If the goal is a bigger kitchen or open-plan living area and you already have enough bedrooms, a single storey does the job without the extra cost.

• Budget is tight. The £20k–£30k difference between single and double storey is significant if you’re stretching.

• You want to avoid planning permission. A single storey under PD can start much sooner.

• Neighbour sensitivity. A double storey has much more visual impact on neighbours’ light and outlook, making objections more likely.

When a Double Storey Is the Better Investment

• You need bedrooms AND living space. If you’re short on both, a double storey solves two problems at once.

• Cost efficiency matters. The cost per m² is substantially lower for a double storey. If you’re going through the disruption of foundations and groundwork anyway, it makes financial sense to build upward.

• You’re building for the long term. If you plan to stay in the property for 10+ years, the extra investment in a double storey usually pays for itself through added value and livability.

• Garden space is limited. Both options have the same footprint. A double storey gives you twice the space without using any more garden.

A Practical Middle Ground

Some homeowners opt for a single-storey extension now with the foundations and structure designed to support a second storey later. This costs a few thousand extra upfront (deeper foundations, stronger walls) but gives you the option to add the upper floor in the future without starting from scratch. It’s worth discussing with your architect if you think you might want the extra floor eventually.

Weighing up your options? We build both single and double storey extensions across Christchurch and Dorset. Come and talk to us about what you need — sometimes the right answer only becomes clear when someone experienced looks at your actual property.