Double-Storey Extension London: Planning, Costs and What to Expect

When a family needs more space, the instinct is often to consider moving house. A double-storey extension changes that calculation significantly.

Consider the comparison: a single-storey extension of 4m x 5m adds 20m² of floor area. A double-storey extension of the same footprint adds 40m² — at a total cost of approximately £90,000–£130,000 versus the £55,000–£75,000 for the single storey. You get twice the floor area for approximately 60–70% more cost. The cost per additional square metre drops from approximately £3,000–£3,500/m² to approximately £2,200–£2,800/m².

In London, where property values are high, the value of additional floor area is correspondingly high. An estate agent valuation of a well-extended three-bedroom terrace versus the same property with five bedrooms (added via a double-storey extension plus loft) may show a value differential of £200,000–£400,000+ in many London postcodes.

The economics strongly favour double-storey extensions in most London scenarios — but only if planning permission can be obtained and the design can be executed correctly.

Double-storey rear extensions almost always require planning permission. The permitted development rules for rear extensions (Class A) only allow single-storey rear extensions up to a depth of 4m (detached) or 3m (semi/terrace) without permission. Any rear extension with upper-floor elements — even a modest one — falls outside this and requires a planning application.

Key planning policy considerations for double-storey extensions in London:

Rear garden depth: most London boroughs require the remaining rear garden to be a minimum depth after the extension. Islington and Camden typically require at least 7–10m of remaining garden. Southwark and Hackney may require 5m+. If the garden is short, a double-storey extension may be refused on overdevelopment grounds.

Daylight and sunlight: a two-storey structure casts a larger shadow than a single-storey one. Planning officers assess the impact on the daylight and sunlight reaching neighbouring properties. Extensions that would cause significant daylight loss to a neighbour's principal rooms are commonly refused.

Design and materials: double-storey extensions must be designed to complement the existing property. A pastiche or clash of materials will attract comment. Some boroughs require extensions to be clearly subordinate to the main house in terms of height, width, and material.

Pre-application advice: for any double-storey extension in London, pre-application advice from the local planning authority is strongly recommended. The cost is modest (£50–£500) and the feedback can save significant architect fees if a proposed design has fundamental policy issues.

Application timescale: 8–13 weeks in most London boroughs from valid application to decision. Delays are common — budget 12–14 weeks.

A double-storey extension involves more structural complexity than a single-storey one, primarily because the upper floor structure must be designed and built correctly.

Foundations: a two-storey extension puts more load on the ground than a single storey. Strip foundations for a double-storey extension are typically 600–900mm wide and 1m+ deep (deeper if near trees or on clay, which is common in London). Piled foundations may be required on made ground, former fill sites, or where trees are present — adding £5,000–£15,000 to foundation costs.

External walls: both floors are typically brick-and-block cavity construction, matching the existing house. Larger extensions may use timber frame for the upper floor to reduce weight and speed construction — less common in London but used where weight loading onto foundations is a consideration.

First floor structure: the floor between ground and first extension floors is typically engineered joists or concrete. Engineered joists (I-joists or metal web joists) span longer distances than traditional timber joists and are the standard specification in modern extensions.

Structural openings: connections from the extension into the existing house are required at both ground and first floor levels. Two structural steels, two sets of temporary support, two sets of padstones and piers. This roughly doubles the structural opening cost compared to a single-storey extension — budget £12,000–£22,000 for both openings combined.

Roof: the double-storey extension roof is typically pitched (matching the existing house) or flat (EPDM). Pitched roofs integrate better architecturally and are often required by planning. They also provide better thermal performance at the upper floor.

The most common configuration for a double-storey rear extension in London:

Ground floor: open-plan kitchen-diner. The existing rear wall is partially or fully removed to connect the extension to the main house. Bifold or sliding doors open to the garden. This becomes the primary family living space.

First floor: master bedroom or additional bedroom. This floor connects through the first floor rear bedroom of the existing house — which may require reconfiguring to create the connection. An en-suite bathroom at this level is highly desirable and should be planned from the outset.

• Alternative configurations:
• •Ground floor utility room + snug; first floor bedroom + study
• •Ground floor playroom or reception; first floor two bedrooms (for large extensions)
• •Ground floor open-plan; first floor master suite with dressing room and wetroom

Width: most double-storey extensions in London run 3.5–6m wide, limited by the plot width and planning constraints. Full-width extensions (matching the full width of the main house) are the most common in modern planning applications where the existing side returns are being incorporated.

Depth: 3–5m is typical for double-storey extensions in London. Beyond 5m, daylight impact on neighbours becomes more significant and planning refusal rates increase.

Connection to existing staircase: one of the most common design challenges is how the first floor of the extension connects to the existing first-floor landing. This usually requires the rear bedroom to be sacrificed or reconfigured — which must be modelled carefully to ensure the net bedroom gain of the extension is not cancelled out by the bedroom loss.

The total cost varies significantly by size, specification, and site constraints.

Small double-storey (3m x 3.5m each floor): both floors combined approximately 21m². Basic spec, no bathroom at first floor: £78,000–£92,000.

Standard double-storey (4m x 4.5m each floor): both floors combined approximately 36m². Kitchen at ground floor, bedroom at first floor, no en-suite: £92,000–£115,000.

With en-suite at first floor: add £10,000–£18,000. Total: £105,000–£130,000.

Larger extension (5m x 5m each floor) with en-suite: both floors combined approximately 50m². £120,000–£155,000.

Premium specification (bespoke kitchen, luxury wetroom, zinc roof, aluminium windows throughout): £145,000–£200,000+.

• Additional costs:
• •Planning application + architect design fees: £4,000–£9,000
• •Structural engineer: £2,000–£4,000
• •Party wall surveyor fees: £700–£1,500 per adjoining owner
• •Second-fix materials (client-supplied): £5,000–£25,000 depending on spec
• •Kitchen (separate budget): £8,000–£60,000+
• •VAT at 20% on all build costs

Timeline on site: 14–20 weeks for a standard double-storey extension in London.

A double-storey extension almost always involves party wall matters — the foundation excavation and the construction of two storeys of external wall on or near the boundary both trigger the Party Wall etc. Act 1996.

Section 6 (excavation within 3m of a neighbouring structure): foundation excavation for a double-storey extension typically falls within 3m of the neighbouring property's foundations, especially in a typical London terrace or semi where the properties are close. A Section 6 notice must be served on any owner of a property within 3m of the excavation.

Section 2 (work on the party wall itself): if the extension wall incorporates the existing party wall or sits against it, Section 2 notices are also required.

In a semi-detached or end-of-terrace London property with one adjoining neighbour, budget £700–£1,500 for party wall surveyor fees if the neighbour dissents to the notice. In a mid-terrace with two neighbours, budget £1,400–£3,000.

Party wall notices cannot be served until the design is sufficiently advanced to describe the works accurately. Factor the 2-month notice period into the project programme — serving notices concurrent with the planning application is the most efficient sequencing.