Polished Concrete and Screed Floors for London Extensions 2025: Specification and Costs

**The fundamental distinction: polished existing slab vs. polished screed overlay**

The term 'polished concrete' is used loosely to describe several related but technically distinct floor finishes:

**1. Polished in-situ structural concrete slab (true polished concrete)**:

The structural concrete slab (the extension's ground floor slab) is ground and polished in place, without any additional overlay or topping. This is the 'true' polished concrete finish — the slab that forms the structural floor of the extension is also the finished floor surface.

• For this to work as a high-quality finished floor:
• •The structural slab must be poured specifically for polishing — the concrete mix design, pour consistency, and finishing (the slab surface must be power-floated or hand-trowelled to a specific smoothness before curing) must be correct from the start
• •The concrete design mix must include the right aggregate type and density for the desired visual result after grinding. The aggregate exposed by grinding is what gives polished concrete its character — river gravel (natural round aggregate) gives a pebble-like appearance; crushed granite gives a sharper, more geometric look; white marble aggregate gives a lighter, more terrazzo-like result
• •The slab must be a minimum of 100–150mm thick and reinforced adequately to take the grinding loads and vibration
• •Any surface defects (cracks; voids; honeycombing from poor pour technique) will be exposed and amplified by grinding — and while crack repair with matching epoxy is possible, the repair will usually be visible in a polished finish

For most London single-storey rear extension projects where a standard structural concrete slab is poured by a general contractor (without specific polishing intent), the slab quality is rarely suitable for polishing without a topping screed. The structural slab is designed to structural standards, not aesthetic finishing standards — pour consistency, surface flatness, and aggregate distribution are not controlled for polished concrete finish requirements.

**2. Concrete topping screed (bonded or unbonded; polished)**:

A concrete topping screed is a layer of concrete or cementitious mortar (typically 65–100mm deep) laid over the structural slab and polished after curing. This is the most common approach for polished concrete floors in new London extensions:

• •**Mix design**: a dry-pack mortar or self-compacting concrete (SCC) with controlled aggregate type, maximum aggregate size, and cement content. The mix is designed specifically for the polishing process — aggregate distribution must be consistent across the slab for a uniform finished appearance
• •**Reinforcement**: typically steel fibre reinforcement (randomly distributed steel fibres added to the mix) or light steel mesh (A142 or A193) to control cracking. Steel fibres produce more uniform crack control than mesh for a topping screed
• •**Minimum depth**: 65–80mm for a standard polished screed. Below 65mm, the risk of 'map cracking' (a fine network of shallow cracks over the surface) increases, and the slab may not have sufficient depth to hide any colour variation at the aggregate level during grinding
• •**Bonding to the structural slab**: a bonded screed (the topping is mechanically bonded to the existing structural slab using a bonding agent and a keyed surface) reduces the risk of differential movement between the slab and the topping. An unbonded screed (with a slip membrane between the structural slab and the topping) can be used where the structural slab surface is not suitable for bonding, but requires a minimum 75mm depth and is at slightly higher risk of edge curl

**3. Micro-cement (microtopping or microtopping screed)**:

• Micro-cement (Ardex Pandomo; Toupret Microcement; Mapei Mapecoat; and similar products) is a thin, polymer-modified cementitious overlay applied in 2–4mm total thickness over an existing floor surface. It provides a grey-tone smooth finish that visually resembles polished concrete but is NOT polished concrete:
• •Applied with a trowel in 2–3 coats (1–2mm per coat); sealed with a polyurethane or epoxy sealer
• •The thin application means it cannot expose aggregate — it has a plain, smooth, solid-colour appearance
• •Suitable for application over existing sound concrete, tile, terrazzo, or plywood subfloors with appropriate primer and bonding agent
• •Does NOT produce the exposed aggregate visual character of polished concrete
• •2025 supply and installation cost: £50–£90/m² for microtopping over existing substrate (2–4mm depth); including 2-coat polyurethane sealer

Micro-cement is the right choice where: the floor is an existing hard finish that cannot be raised significantly; the client wants a smooth concrete-look floor with minimal height gain; or the budget does not support a full screed and polish.

**4. Terrazzo (polished aggregate and cement matrix)**:

Terrazzo is a composite floor material consisting of marble, granite, quartz, or glass chips set in a cement or epoxy matrix, poured or placed in a layer, and then ground and polished to a smooth surface. Terrazzo is distinct from polished concrete — the aggregate is decorative and deliberately specified (marble chips; glass chips; brass dividers between colour fields) rather than the structural aggregate of the concrete slab. Terrazzo is experiencing a significant design revival in London residential projects. Costs: £85–£200/m² supply and installation depending on aggregate type and field pattern complexity.

**The four phases of the grinding and polishing process for a concrete topping screed**:

*Phase 1 — Cure and preparation*:

The concrete topping screed must cure for a minimum of 28 days before grinding begins. During curing, the screed gains strength and the aggregate distribution stabilises. Any plastic shrinkage cracks (hairline surface cracks that appear in the first 24 hours due to rapid surface moisture loss) should be assessed — very fine plastic shrinkage cracks are acceptable and will be ground away in the coarse grinding phase; wider cracks or structural cracks must be investigated and repaired before grinding.

After curing, the surface may be treated with a lithium silicate densifier (see Phase 3 below) or this may happen later in the process — the specifier and polishing contractor's preference varies.

*Phase 2 — Coarse grinding (abrasive cut; aggregate exposure)*:

The polishing machine (a walk-behind planetary grinder; typically 450–650mm wide) is equipped with coarse diamond tooling (typically 30-grit or 16-grit metal-bonded diamond segments for the initial cut). The machine grinds the surface of the screed, removing the surface 'cream' (the paste layer of cement and fine particles at the slab surface) and beginning to expose the aggregate below.

• The depth of grinding determines how much aggregate is exposed:
• •**Salt-and-pepper finish** (minimum grind; top 1–2mm removed): the surface cream is removed and a small amount of aggregate visible — the appearance is specked rather than a full aggregate reveal
• •**Medium exposure finish** (top 2–4mm removed): more aggregate visible; the aggregate is clearly the dominant visual element
• •**Full aggregate exposure** (top 4–6mm+ removed): the full cross-section of aggregate is exposed; the floor looks like a section through the concrete, with aggregate pieces clearly visible in their full diameter

The grinding passes progress through increasingly fine grit tooling: 30 grit → 50 grit → 100 grit → 200 grit (all metallic-bond diamond segments). At each grit, the machine removes the scratch marks from the previous pass and leaves finer scratches of its own.

*Phase 3 — Densifier application*:

After the 100–200 grit metallic bond phase, a **lithium silicate densifier** (Ashford Formula; HardenerPlus; Curecrete CureHard; and equivalent products) is applied to the surface by spray or roller. The densifier penetrates the open concrete pores and reacts with the free calcium hydroxide (portlandite) in the cement matrix to form calcium silicate hydrate (CSH) — filling the pores, hardening the surface, and making it resistant to further moisture ingress. The densifier is allowed to penetrate for the manufacturer's recommended time (typically 20–30 minutes) before excess is removed and the surface buffed.

Densifier application is a critical step — it raises the surface hardness significantly and is what allows the following resin-bond polishing phase to achieve a true gloss without scratching the surface. Skipping the densifier produces a softer, duller, more porous finish that stains more easily.

*Phase 4 — Resin-bond polishing (polish phase)*:

• After densifier application, the machine heads are changed from metallic-bond diamond segments to **resin-bond diamond** pads. Resin-bond pads produce finer surface finishes than metallic-bond segments:
• •400 grit resin: satin finish (some reflection; no clear image reflection)
• •800 grit resin: semi-gloss (directional reflection beginning to appear)
• •1500 grit resin: gloss (clear image reflection)
• •3000 grit resin: high gloss (clear, mirror-like reflection with full image)

• The commonly specified finish levels for London residential extensions:
• •**Level 2 (coarse grind; salt-and-pepper; no gloss)**: aggregate partially exposed; matte finish; typical for industrial-character spaces or where a low-maintenance easy-to-spot-clean surface is priority. Stops after metallic bond phase.
• •**Level 3 (medium aggregate exposure; satin 400–800 grit resin)**: the most common residential specification. Good aggregate reveal; satin sheen; easy to maintain; does not show foot traffic marks as readily as high gloss.
• •**Level 4 (high gloss; 1500–3000 grit resin)**: full aggregate exposure; mirror finish; impressive in natural light; requires more regular maintenance (resealing; buffing) to maintain the appearance. Highly visible foot traffic marks on dust or fine particles.

*Final sealer application*:

• After polishing, a penetrating or topical sealer is applied. Options:
• •**Lithium silicate densifier** (already applied in Phase 3): provides hardness and moisture resistance but does not add surface sheen — appropriate for Level 2–3 finishes where the shine comes from the grinding, not from a surface layer
• •**Penetrating silane/siloxane sealer**: provides water and oil repellency; does not significantly change the appearance
• •**Topical polyurethane or epoxy sealer** (for Level 3–4 finishes): applied as a thin topcoat; adds a defined level of sheen; provides additional stain resistance; must be re-applied every 3–5 years as it wears

For London kitchen extension floors (food preparation; potential spillages; children's play): a penetrating sealer providing oil and water repellency is recommended. A topical sealer adds another layer of stain protection but will show wear in high-traffic areas and must be maintained.

**Polished concrete and underfloor heating — an excellent combination**:

Polished concrete is one of the best floor finishes to use above wet underfloor heating (UFH). Concrete has high thermal mass and good thermal conductivity — it absorbs heat from the UFH pipes efficiently and re-radiates it slowly and evenly into the room. Key specification points where UFH and polished concrete floors are combined:

• •**UFH pipe depth below the polished surface**: the UFH pipes (PEX-a; 16mm or 20mm diameter; typically at 150mm or 200mm centres) must be below the level at which the polishing grinding will cut. The concrete topping screed must be thick enough to:
• a) Accommodate the pipe at the correct cover depth (minimum 25mm concrete cover over the pipe to the surface before grinding)
• b) Allow for the grinding depth (3–6mm removed from the surface during grinding)
• Total minimum screed depth above pipe: 25mm cover + 6mm grinding depth = 31mm minimum above pipe crown. With a 20mm pipe, the pipe centre is 10mm above the insulation board, so the screed depth from pipe crown to finished surface must be at least 31mm — meaning the screed is poured to minimum 75mm depth (10mm pipe crown height + 25mm cover + 6mm grind + 34mm screed above for structural integrity)
• In practice, specify a minimum 80–100mm deep topping screed where UFH and polished concrete are combined

• •**Aggregate type and UFH performance**: the aggregate type does not significantly affect UFH thermal performance — thermal conductivity of the concrete matrix dominates. Specify aggregate for visual effect (river gravel for round soft look; crushed granite for angular sharp look; marble aggregate for lighter terrazzo look)

• •**Screed mix design for UFH**: use a mix with good thermal mass (dense aggregate; controlled water:cement ratio of 0.4–0.45 for strength). Avoid lightweight aggregate for UFH topping screed — it reduces thermal conductivity

• •**Heat commissioning before polishing**: after the screed is poured and cured (minimum 28 days), the UFH must be heat-commissioned (gradual warm-up to operating temperature over 7–10 days) BEFORE grinding and polishing. This causes any residual moisture to evaporate and stabilises the screed dimensionally. Starting the UFH before grinding also reveals any weak areas of the screed that may need repair before polishing proceeds.

**Structural considerations for polished concrete in London extensions**:

• A polished concrete topping screed (80–100mm deep) has a significant weight. At a typical density of approximately 2,200–2,400 kg/m³:
• •80mm deep concrete topping: approximately 176–192 kg/m²
• •100mm deep concrete topping: approximately 220–240 kg/m²

For a new extension on a structural concrete ground-bearing slab: the additional weight of the topping screed is typically not an issue — the extension slab and its foundation are designed for the relevant dead and live loads, and the structural engineer can incorporate the topping screed load in the original structural design.

For an existing first floor (a polished screed over an existing timber or concrete upper floor): the existing floor structure may not be designed for the additional weight. A structural engineer's assessment is required before specifying a polished concrete topping on any upper floor or existing floor that was not designed for it. Do not assume a first-floor extension or existing upper floor can take a polished concrete topping without a structural check.

**2025 polished concrete and screed floor costs for London extension projects**:

| Scope | Unit | Cost range (London 2025; supply + installation) | |---|---|---| | Concrete topping screed pour (80mm; including mix design and steel fibre reinforcement) | per m² | £55–£90/m² | | Level 2 finish (coarse grind; salt-and-pepper; matte; densifier) | per m² | £25–£40/m² | | Level 3 finish (medium aggregate; satin 400–800 grit; densifier + penetrating sealer) | per m² | £40–£70/m² | | Level 4 finish (full aggregate; high gloss 1500–3000 grit; densifier + topical sealer) | per m² | £65–£120/m² | | Micro-cement (microtopping; 2–4mm over existing substrate; polyurethane sealer) | per m² | £50–£90/m² | | Crack repair (epoxy injection + matching colour repair; per linear metre) | per m | £20–£60/m | | Resealing (topical polyurethane; per m²; periodic maintenance) | per m² | £8–£20/m² |

• *Typical total cost for a polished concrete topping screed floor in a 30m² London kitchen extension (above structural slab; Level 3 satin finish; including UFH-compatible 80mm screed pour + 400–800 grit polish + densifier + penetrating sealer)*:
• •Concrete topping screed pour: 30 × £70 = £2,100
• •Level 3 polish: 30 × £55 = £1,650
• •Densifier and sealer: 30 × £12 = £360
• •Crack repair allowance: £200
• •**Total: approximately £4,300–£5,500** for a 30m² polished screed extension floor (Level 3 finish; UFH-compatible specification)

*By comparison, the same 30m² floor with large-format porcelain tiles (600×600mm; mid-range tile; adhesive and grouting): approximately £1,800–£3,500 — polished concrete is 50–100% more expensive than mid-range porcelain tile installation, which reflects the additional screed preparation, specialist grinding equipment, and multi-phase polishing process.*