Single Storey vs Two Storey Extension: Cost and Planning Comparison

Single Storey vs Two Storey Extension: Cost and Planning Comparison

Choosing between a single storey and a two storey rear extension is one of the most consequential decisions in home renovation. The planning rules, structural requirements, costs per square metre, and impact on neighbours differ significantly between the two options. This guide gives you a clear framework — including specific costs, PD limits, and the key factors that make one option better than the other for your situation.

Cost Comparison: The Numbers That Matter

Contrary to what many people assume, a two storey extension typically costs less per square metre than a single storey extension. The reasons are straightforward: you share the foundation costs, scaffolding costs, and roof costs across double the floor area.

The critical insight: if you build a two storey extension of 30 m² per floor, you get 60 m² of new space at a total cost of £90,000–£168,000 — roughly the same price as a premium 30 m² single storey extension. Where planning and household need allow, two storey provides substantially better value per m².

Costs at the higher end of each range apply in London and the South East. Midlands and Northern England typically sit 20–30% below.

Single Storey Extensions: Permitted Development Rules

Single storey rear extensions in England fall under Class A, Part 1 of the GPDO 2015. Under standard permitted development:

• Attached houses (semi-detached, terraced): maximum 3 metres depth from the original rear wall.
• Detached houses: maximum 4 metres depth from the original rear wall.

Under the Neighbour Consultation Scheme (NCS) — also called Prior Approval — you can extend further:

• Attached houses: up to 6 metres depth.
• Detached houses: up to 8 metres depth.

The NCS process: notify your local planning authority, who consults immediately adjoining neighbours. If no objection is raised within 42 days, consent is granted. The fee is £120. If neighbours do object, the LPA assesses on limited grounds — amenity and appearance — not general planning policy.

Additional PD conditions for single storey extensions:

• Maximum eaves height of 4 metres.
• No extension beyond the principal elevation.
• Materials to match the existing house in appearance.
• No side extensions on designated land or in conservation areas.

Two Storey Extensions: Permitted Development Rules

Two storey rear extensions are not included in the Neighbour Consultation Scheme. The standard Class A PD limits for two storey structures are significantly more restrictive:

• Maximum 3 metres depth from the original rear wall.
• Must not be within 7 metres of the rear boundary.
• Must not exceed the height of the existing roof ridge.
• Must comply with the 45-degree rule: no part of the extension should extend beyond a 45-degree line drawn from the nearest window of a habitable room in an adjoining property.
• Side elevations of two storey extensions must not have windows that overlook neighbouring gardens unless obscure-glazed.

In practice, most two storey rear extensions that extend more than 3 metres, or involve significant bulk at first-floor level, will require full planning permission. This is the most important practical distinction between the two options.

Planning Application Process for Two Storey Extensions

Where full planning permission is required, the process involves:

1. Pre-application advice (optional but valuable): £50–£200 from the LPA. Provides early feedback before you invest in detailed drawings.
2. Architectural drawings: Site plans at 1:1,250 and 1:500, floor plans and elevations at 1:50 or 1:100.
3. Householder application: Submitted via the Planning Portal. Fee: £258 in England (2024).
4. Determination period: 8 weeks statutory target, though many LPAs take 10–14 weeks.
5. Key considerations: Overlooking from new upper-floor windows, loss of daylight to neighbouring habitable rooms (BRE 209 methodology), bulk and massing in the streetscene, materials.

A refused application can be appealed to the Planning Inspectorate — but this adds 3–6 months and legal costs. Better to address likely concerns in the design stage.

Structural Differences

Foundations A two storey extension requires deeper, more heavily reinforced strip foundations than a single storey. Foundation depth typically increases from 600–750mm (single storey, good ground) to 900mm–1.2m (two storey, variable ground). Typical additional foundation cost: £2,000–£6,000.

Structural steels Spanning the opening between the new extension and the existing house at ground floor typically requires one or two RSJ steel beams. At first-floor level, a further steel beam is usually required to carry the new floor over the ground-floor opening. A structural engineer's calculations are required for Building Control approval. Typical fees: £300–£600 per beam calculation, or £800–£2,000 for a complete two storey structural package.

Party Wall Act Two storey extensions more commonly trigger the Party Wall etc. Act 1996 than single storey, because the deeper foundations required by Section 6 (excavations within 3 metres of a neighbouring building's foundations) and the greater wall height. Budget for £700–£2,500 in party wall surveyor fees depending on the complexity.

Scaffolding Two storey work requires full scaffold at both levels — a modest additional cost relative to the total, but worth factoring in (£1,500–£3,500 more than single storey scaffolding).

Return on Investment

Extensions add value by increasing usable floor area and, more specifically, bedroom count. Typical value uplift:

• A large, open-plan kitchen-diner-living room: 5–10% uplift (high demand in current market).
• An additional bedroom (single storey or first floor of two storey): 7–12% uplift.
• Two bedrooms added (two storey): 10–18% uplift in the right property and location.

ROI depends heavily on local house prices. In high-value areas (inner London, commuter belt), extensions frequently recoup their cost in added value. In lower-value areas, the ROI on premium extensions is often negative in pure financial terms — but the lifestyle value is real.

When Two Storey Makes More Financial Sense

Choose a two storey extension when:

• Your household needs both additional ground-floor living space and additional bedrooms.
• The cost per m² advantage matters to you — you get more space for the same budget.
• Planning permission is achievable (your neighbours are unlikely to raise strong objections; overlooking can be mitigated by window design).
• The property has strong value fundamentals that can absorb the build cost.

Choose a single storey extension when:

• Your primary need is open-plan kitchen-diner-living space.
• PD rules allow a large single storey extension without planning permission, avoiding the delay and uncertainty of a full application.
• The property is in a conservation area or has restricted planning history making two storey consent uncertain.
• Neighbours would face significant overlooking from first-floor windows that cannot be adequately mitigated.

Typical Timelines

Impact on Neighbours: Overlooking and Daylight

The 45-degree rule is the primary planning test for both extension types. For two storey extensions, the first floor adds a direct overlooking risk to neighbouring rear gardens and habitable rooms that is absent in a single storey scheme.

Mitigations that planners typically accept include:

• Obscure glazing on side-facing windows at first floor.
• High-level windows (above 1.7m from finished floor level) on the rear elevation closest to the shared boundary.
• Restricting the number of windows on the shared-boundary elevation.

A well-designed two storey extension incorporates these measures from the outset — this satisfies the LPA, prevents refusal, and maintains good neighbour relations throughout the build.

How Renovate Me Helps

Renovate Me's renovation roadmap takes you from initial idea to completed extension — helping you assess which option suits your plot and planning context, understand your PD rights, prepare a planning application that addresses overlooking concerns upfront, and manage the structural engineering and party wall obligations that determine whether a two storey project stays on time and on budget.

For comprehensive cost data by extension type and region, see our House Extension Costs UK 2025 guide. For the full householder planning application process — including fees, drawings, conservation area rules, and what to do if refused — see our Planning Permission for a House Extension UK guide.