Single-Storey vs Double-Storey Extension: Which Is Right?

Extending out or extending up is the first real fork in most extension projects, and it changes the budget, the planning route and how long you live with scaffolding. This guide compares a single-storey vs double-storey extension on the numbers that matter: cost per m² and in total, with worked examples, the permitted development rules for each, build times, the value each adds, and a straight checklist for choosing. Figures are for England in 2026, exclude VAT unless stated, and are based on our own quotes and published UK trade guides.

The quick answer

• Single-storey is the cheaper project in total, the more likely to be permitted development, and the fastest to build. Choose it if what you want is a bigger kitchen-diner and a connection to the garden.
• Double-storey is cheaper per square metre and adds a bedroom and bathroom, which moves a house into a different price bracket. Choose it if you need space upstairs as well as down and can accept a planning application and a longer build.
• If you need both and can fund it, build both floors at once. Adding a first floor later costs far more than doing it now.

What is the difference?

A single-storey extension adds ground-floor space only: a rear extension across the back of the house, a side-return on a terrace, or a wrap-around combining both. It usually becomes a kitchen-diner or family room and, on the right house, can be built with a flat or pitched roof and roof lights.

A double-storey extension, also called a two-storey extension, adds a ground-floor room with a room above it, typically a bedroom, often with an en-suite. The first floor joins the existing landing, and the roof is tied into the main roof, which is why it is a more involved structure.

Both can be rear, side or wrap-around. A side extension on a semi is usually single-storey because permitted development only allows single-storey side extensions, and only up to half the width of the original house; a double-storey side extension needs planning permission.

Double-storey extension cost vs single-storey

Cost per m² in 2026

The second storey is cheaper per m² because the expensive fixed items are shared: foundations, groundworks, drainage, the roof and the structural connection to the house. Scaffolding is the one cost a double-storey build adds that a single-storey one often avoids.

Worked example: a 4 m x 5 m footprint

Take the same 20 m² footprint on a 1960s semi in Hampshire:

• Single-storey kitchen extension: build £45,000 to £65,000, kitchen £15,000 to £25,000, glazing £5,000 to £8,000, fees and approvals £4,000 to £6,000. Total £69,000 to £104,000 ex VAT. Sixteen weeks on site.
• Double-storey extension, kitchen below and bedroom with en-suite above: build £75,000 to £105,000, kitchen £15,000 to £25,000, bathroom £8,000 to £14,000, glazing £6,000 to £10,000, scaffolding £3,000 to £5,000, fees, planning and party wall £6,000 to £10,000. Total £113,000 to £169,000 ex VAT. Twenty to twenty-two weeks on site.

The second storey adds another 20 m², doubling the floor area to 40 m², for around 60 per cent more money, which is where the per-m² saving comes from. Our house extension cost guide breaks every line down further.

Why double is not double the price

Foundations and groundworks are dug once. The roof is built once. The drainage connection and the knock-through into the house happen once. The engineer designs one structure. Only the walls, floor, windows, first fix and fit-out scale with the second floor, and on the upper floor there is no expensive kitchen, just a bedroom and, usually, a bathroom.

What the per-m² figure leaves out

VAT at 20 per cent, professional fees, the planning fee, party wall costs on a semi or terrace, kitchen and bathroom fit-out, and the steel to open the existing house into the new space. Add 10 to 15 per cent contingency to either option.

Planning permission and permitted development

Single-storey extensions

A single-storey rear extension is permitted development in England if it extends no more than 3 metres from the original rear wall on an attached house or 4 metres on a detached house, is no more than 4 metres high, has eaves no higher than 3 metres within 2 metres of a boundary, and stays within the other conditions on materials and coverage. The larger home extension scheme lifts the depth to 6 and 8 metres with prior approval, a 42-day neighbour-consultation process. Wrap-arounds usually need a full application.

Double-storey extensions

A two-storey rear extension can also be permitted development, but the limits are tighter: no more than 3 metres beyond the original rear wall, at least 7 metres from the rear boundary, a roof pitch matching the existing house as far as practicable, and any upper-floor side windows obscure-glazed and non-opening below 1.7 metres. None of this applies on designated land, so in a conservation area, a national park or an Area of Outstanding Natural Beauty a two-storey extension always needs planning permission. On a typical suburban plot the 7-metre rule is the one that catches people, which is why most double-storey extensions go through a full householder application.

Fees and the application

A householder application costs £548 in England in 2026 plus a Planning Portal service charge of around £90, and the council has eight weeks to decide once the application is validated. A lawful development certificate for a permitted development extension costs £274 and is worth having. Our guide to planning permission for extensions covers the full rule set, refusals and what happens if you build without consent.

Build time and disruption

You can usually live in the house during either build. The single-storey extension disrupts the kitchen at the end; the double-storey extension also opens the landing to join the new bedroom, which is the noisiest and dustiest week upstairs. Both benefit from ordering the kitchen, windows and doors before work starts, because glazing lead times run four to ten weeks.

Is a double-storey extension worth it?

Usually, yes, if you need the bedroom. A bedroom and en-suite move a three-bedroom house into the four-bedroom bracket, which is why, as a rule of thumb, double-storey extensions typically add 10 to 20 per cent to value, more than a single-storey extension of the same footprint. The two limits are the ceiling price of your street, beyond which further spending does not come back, and the quality of the design: a double-storey extension that overshadows the garden or sits awkwardly against the original roof adds less than a well-proportioned one. An estate agent who knows the road will tell you in ten minutes what extended houses actually fetch.

A single-storey extension adds value roughly in line with the floor area it creates, and an open-plan kitchen-diner is the single most sought-after feature in family houses, so it is rarely wasted money either.

Design considerations

• Light. A deep single-storey extension can leave the middle of the house dark; roof lights or a roof lantern fix it. A double-storey extension cannot use roof lights on the ground floor, so side windows and big rear glazing matter more.
• Overlooking and the 45-degree rule. Upper-floor windows in a double-storey extension are what neighbours object to. Planners test loss of light with a 45-degree line from the neighbour's nearest window; design around it early.
• Matching the house. Brick, mortar colour, roof pitch and tiles, window proportions. A double-storey extension is judged against the existing house from the street and the garden; a poor match is the most common reason they look bolted on.
• Structure. Both need a structural engineer for the steel opening into the house; a double-storey extension also needs the first-floor structure and roof designed and the existing roof tied in.
• Stairs and landing. The new bedroom has to connect to the landing. On some houses that means losing a small bedroom or a cupboard to the corridor; check this before falling in love with the plan.

When to choose single-storey

• The brief is a kitchen-diner, family room or connection to the garden
• You want the fastest, least disruptive build
• Budget is £60,000 to £130,000 all in
• The house is on a small plot where a two-storey rear wall would dominate the garden
• You want to stay within permitted development where possible
• You plan a loft conversion for the extra bedroom instead

When to choose double-storey

• You need a bedroom, and ideally an en-suite, as well as downstairs space
• The plot is deep enough to keep 7 metres to the rear boundary, or you are ready for a planning application
• You want the best value per square metre and can fund the larger total
• The house sits well below the ceiling price of the street
• You would rather do one build than two

Extensions in Southampton: single or double?

Around Southampton the housing stock pushes the decision one way or the other. The 1960s to 80s semis and detached houses of Hedge End, Locks Heath, Totton and North Baddesley sit on plots that often take a double-storey rear extension within the 7-metre rule. Victorian terraces in Southampton, Eastleigh and Bishopstoke are single-storey side-return territory, with a loft conversion for the bedroom. Conservation areas in Winchester, Romsey and parts of Fareham borough take the permitted development option off the table for two-storey work. Our featured Locks Heath kitchen extension is a single-storey rear extension, chosen because the brief was an open-plan kitchen-diner and the bedrooms were already there.

A third option: building over an existing extension or garage

If you already have a single-storey extension or an attached garage, a first floor on top of it can deliver the bedroom without touching the garden. The catch is the structure underneath: garage and older extension foundations were rarely built for two floors, so an engineer checks them with a trial hole before anything is designed. Where they pass, expect £40,000 to £90,000 for a bedroom-sized room with the existing roof removed, the walls strengthened and tied in, and a new roof; where they fail, underpinning at £10,000 or more usually makes it cheaper to build a new double-storey extension instead. Over-garage rooms also need proper insulation to the garage below and fire separation, which building control will look for.

The right answer depends on the plot, the planning position and what you actually need the space for. Prices in this guide are based on our own quotes and published UK trade guides and are correct as of August 2026.