Single vs Double-Storey Extension: Which Is Right for Your Home?

The Decision Most Homeowners Get Wrong

Most homeowners start with a single-storey extension because it feels like the simpler, cheaper option. Sometimes it is. But for a growing family that needs both more ground-floor living space and an additional bedroom, building single-storey now and double-storey later costs significantly more than building double-storey from the outset. Foundations, scaffolding, and planning are not costs you want to pay twice.

The right choice depends on what you need, what your property allows, and what your budget can accommodate. This guide works through each factor directly.

Cost Comparison

A single-storey rear extension in Hertfordshire costs £40,000–£95,000 depending on size and specification. A double-storey rear extension of the same footprint costs £80,000–£180,000. The absolute cost is higher for double-storey, but the cost per square metre is lower — typically £1,400–£2,200/m² versus £1,800–£2,800/m² for single-storey.

The reason is straightforward. Foundations, scaffolding, and the roof are the three most expensive elements of any extension. In a single-storey extension, those costs are spread across one floor. In a double-storey extension, they are spread across two floors. The second floor adds masonry, floor structure, internal staircase modifications, and additional plumbing and electrical work — but it does not add a second set of foundations, scaffolding, or a roof.

If you need both ground-floor living space and an additional bedroom, building double-storey from the outset saves £20,000–£40,000 compared to building single-storey now and adding the second floor later. Retrofitting a second floor onto an existing single-storey extension requires new foundations (the existing ones are not designed for the additional load), new scaffolding, and a new planning application.

Planning Permission

This is the most significant practical difference between the two extension types. Single-storey rear extensions can often proceed under permitted development rights without a planning application, provided they meet the size limits set by the Town and Country Planning (General Permitted Development) Order 2015:

• Detached houses: up to 4 metres deep under standard PD, or up to 8 metres under the Neighbour Consultation Scheme
• Semi-detached and terraced houses: up to 3 metres under standard PD, or up to 6 metres under the NCS
• Maximum height of 4 metres at the eaves and 4 metres at the ridge (pitched roof) or 3 metres (flat roof)

Double-storey rear extensions are not permitted under permitted development rights. A full planning application is required regardless of size. In Hertfordshire, planning applications cost £258 (2025 fee) and typically take 8–10 weeks to determine. Hertsmere Borough Council, London Borough of Barnet, and London Borough of Harrow all have their own design guidance that affects what is likely to be approved.

Properties in conservation areas, listed buildings, and flats require planning permission for both single and double-storey extensions. If your property is in a conservation area — which includes parts of Radlett, Potters Bar, Barnet, and St Albans — the design requirements are more restrictive and the approval process takes longer.

Build Time and Disruption

A single-storey rear extension takes 10–16 weeks from groundworks to completion. A double-storey extension of the same footprint takes 16–24 weeks. The additional time reflects the greater structural complexity, additional masonry work, and the need to complete the first-floor structure before the roof can be installed.

Disruption is higher for a double-storey extension. The rear wall of the house is opened up on both floors, which means bedrooms on the first floor are affected as well as the ground-floor kitchen and living space. TCM installs temporary hoarding and dust barriers on both floors during the structural phase, but the disruption period is longer — typically 4–6 weeks of significant disruption versus 2–3 weeks for a single-storey extension.

Most families remain in the property during both types of extension. The critical period is when the rear wall is opened up — a temporary weatherproof barrier is installed to protect the interior, but the house is noisier and dustier than usual. Planning the project to avoid school exam periods and winter months reduces the practical impact.

Property Value Impact

Both extension types add value to a Hertfordshire property, but the mechanism differs. A single-storey kitchen extension adds value primarily by improving the quality of the most-used room in the house and creating an open-plan space that buyers consistently prioritise. A double-storey extension adds value by increasing the bedroom count — moving from three to four bedrooms is the single most significant value step in the Hertfordshire market.

A well-executed single-storey kitchen extension typically adds 5–15% to a property's value. On a £600,000 semi-detached house in Radlett or St Albans, that is £30,000–£90,000. A double-storey extension adding a bedroom and en suite alongside a kitchen-diner typically adds 10–20%, representing £60,000–£120,000 on the same property.

The value uplift per pound spent is broadly similar for both types. The double-storey extension costs more in absolute terms but adds more value in absolute terms. The decision should be driven by what the household needs, not by which type offers the better theoretical return on investment.

Which Type Is Right for Your Property?

Choose single-storey if: you need more ground-floor living space (kitchen, dining, family room), your budget is £40,000–£80,000, you want to avoid the planning application process, or you do not need an additional bedroom.

Choose double-storey if: you need both more living space and an additional bedroom, your budget is £80,000+, you are prepared for the planning application process, or you want the best value per square metre of new floor area.

Consider a loft conversion instead if: you need an additional bedroom but not more ground-floor space, your garden is small and you do not want to lose outdoor space, or your budget is £40,000–£65,000. A loft conversion adds a bedroom and bathroom without touching the garden and typically falls within permitted development rights for semi-detached and detached properties.