House Extension Design Ideas

The difference between a good extension and a great one rarely comes down to size. It comes down to light, the connection to the garden, and how well the new space integrates with the rest of the house. This guide covers the design moves that consistently deliver the best results, with real costs and practical notes from the construction side.

Making the Most of Natural Light

The single biggest criticism of Victorian and Edwardian terrace extensions is that they sit behind a deep existing house, which means the interior can feel dark and cave-like. There are three reliable tools for solving this:

Rooflights

A flat or slightly pitched rear extension lends itself to rooflights. Glazed panels set flush or near-flush into the roof plane bring in overhead light that no amount of glazing in the rear wall can replicate. Specify:

• Structural glass rooflights (e.g. Glazing Vision, Vision AGI): frameless and thermally broken, with a U-value that depends on the glass specified (check it against the Part L limit below). From £900 per m² of glass, plus structural upstand and installation
• Patent glazing strips (aluminium glazing bars): cost-effective at £350–£550/m² installed; more visible structure but good performance
• Pitched Velux-style units: cheapest at £300–£600 each installed in a flat-to-pitch junction; limited to smaller openings

A 1,000 × 1,500 mm rooflight (installed) costs approximately £600–£1,200 for a standard unit or £1,500–£3,000 for a frameless glass-to-glass unit. Allow one rooflight per 8–10 m² of extension floor area as a rule of thumb.

Sliding and Bifold Doors

Opening the rear wall entirely to the garden is the most transformative move in a ground-floor extension. The options:

Check the U-value against Part L before you sign the order, because several of the systems above will not clear it at the top of their U-value range. A window or door in a new extension has to reach 1.4 W/m²K or better, and a rooflight 2.2 W/m²K.¹ A slim steel system at 1.5–2.0 W/m²K only works if the rest of the extension is specified well enough to carry it through a whole-building calculation, and your designer needs to know that at drawing stage rather than at order stage.

For a rear wall that is load-bearing (common where an original outrigger or return is being opened up), you will need a structural steel beam above the opening. An RSJ or UC section spanning 3.5 m typically costs £1,200–£2,500 installed including padstones and making good.

Glass Box Extensions

A fully glazed side or rear addition using structural glass fins, frameless corner glazing, or a Crittall-style steel frame is the boldest approach. Glass box extensions tend to be designed as a deliberate contrast to the original house, the transparency reads as a light addition that doesn’t try to imitate old masonry. Cost is high (£3,500–£6,000/m² installed for a fully glazed structure) and thermal performance needs careful engineering: overheating in summer is a real risk without a brise-soleil, external blind, or high solar-gain glass.

Blending Old and New Materials

The junction between a new extension and a Victorian or Edwardian terrace is where most projects succeed or fail visually. There are two defensible positions:

Match and blend: Use facing bricks sourced to match the existing hand-made or sand-faced brick. This works best when the extension is clearly subordinate and the plot is in a conservation area where planning will push you toward matching materials anyway. A brick-matching service costs £200–£500; salvage bricks run £1–£3 each compared with roughly £0.70–£1.40 for new facing bricks.

Contrast deliberately: Use a render, untreated timber cladding (larch, Siberian larch, or Douglas fir), or powder-coated steel panel to signal that the extension is a contemporary addition. This is architecturally honest and avoids the uncanny valley of near-matched brickwork. Larch cladding installed (25 × 100 mm open-joint vertical boards) costs £120–£200/m²; K-Rend silicone render costs £45–£80/m².

The roof junction matters as much as the walls. A slim steel or timber fascia board at the transition between the original slate or tile roof and the new flat roof extension looks clean; a bulky parapet or a clumsy lead flashing upstand looks provisional. Specify a 50 mm fascia and a continuous lead soaker or EPDM flashing set into the existing roof, this detail is worth discussing explicitly with your builder before work starts.

Flooring Continuity

Running the same floor finish through from the existing house into the extension makes both spaces feel larger and more intentional. The practical constraints:

• Existing suspended timber floor: The new extension floor (typically concrete slab or beam-and-block) will be at a different structural level. A screed topping on the extension can be brought to the same finished-floor level as the timber in the existing house, allowing a continuous tile or engineered timber run. Allow for a screed, typically 65–75 mm thick, above the insulation and membrane.
• Matching tiles: Porcelain floor tiles (600 × 600 mm, full-body, R10 slip rating minimum) cost £25–£65/m² supply only; large-format 1200 × 600 mm tiles cost £50–£120/m². Underfloor heating under tiles (electric mat or wet system) adds £50–£120/m² installed.
• Engineered timber: A floating engineered oak board (14 mm total, 4 mm wear layer) across old and new floors needs the two floor levels to match closely at the threshold, achievable with careful screed specification.

Layout and Open-Plan Design

The trend toward open-plan kitchen-dining-living at ground floor has been consistent for 15 years and shows no sign of reversing. Extension designs that support it typically:

• Remove the rear reception room wall to merge the existing footprint with the extension, creating one continuous space. This almost always involves a structural beam, budget £2,500–£6,000 for beam supply and installation including making good above and below.
• Zone the open plan with a kitchen island (typically 900–1,200 mm wide × 2,400–3,000 mm long), a level-threshold door to the garden, or a change in ceiling height. A dropped ceiling over the kitchen zone with integrated downlights creates definition without walls.
• Place the kitchen at the rear of the extension, facing the garden. This is the most light-filled position and brings the cook into the social heart of the house. The run from hob to sink to prep area (the kitchen triangle) works well against the rear wall with island seating in front.

Practical Considerations Before You Finalise the Design

• Structural surveys: If you are removing chimney breasts or rear walls, commission a structural engineer’s report before the architect finalises drawings. Survey fees run £500–£1,200 for a single-storey scheme.
• Party Wall etc. Act 1996: Excavating within 3 m of a neighbour’s building and going deeper than the bottom of their foundations triggers a notice, and so does excavating within 6 m where the work cuts a plane drawn down at 45 degrees from the bottom of those foundations.² The notice periods are not all the same, which is where most people go wrong. An excavation notice under section 6 needs at least one month, a line of junction notice under section 1 needs at least one month,³ and only a party structure notice under section 3, for cutting into or altering a shared wall, needs the full two months.⁴ A surveyor’s fee for a straightforward award is £700–£1,500 per side.
• Drainage diversions: Extensions often sit above existing underground drains. A CCTV drain survey before design (£200–£400) prevents expensive design revisions later.
• Ceiling height: There is no minimum ceiling height for a habitable room in the Building Regulations in England. The 2,300 mm figure that gets quoted comes from the nationally described space standard, which asks for 2.3 m over at least 75% of the gross internal area; that is a planning standard for new dwellings, it bites only where a council has adopted it in its local plan, and the government states plainly that it is not a building regulation.⁵⁶ Aim for 2,400–2,700 mm anyway, because it is what makes a room feel right. The lower the ceiling, the more important rooflights become.

A well-designed 20 m² kitchen-diner extension will cost £50,000–£80,000 fully fitted in greater London and the South East in 2026, including architect fees, structural engineer, party wall, building regulations, and kitchen. Architects’ fees are commonly around 8–12% of construction cost, and spending that money properly is the best value-for-money decision you can make.

The rules in this note are those that apply in England. Scotland, Wales and Northern Ireland have their own building standards and permitted development rights.