Polished Concrete Floors London 2025: Cost, Process, and Specification Guide

**The four products commonly called 'polished concrete' — a critical distinction**:

The term 'polished concrete' is used loosely in the UK market to describe at least four distinct products that have very different construction requirements, costs, and performance characteristics:

*1. True polished concrete (ground and polished in-situ)*:

True polished concrete is a structural concrete slab — poured at 75mm minimum thickness (typically 100–150mm in a new extension) — that is ground in stages using progressively finer diamond tooling (from 30 grit to 3000 grit) until the surface is honed and polished to the desired finish. The polishing exposes the aggregate (stone chips; sand; glass; metallic flakes) at various depths depending on the number of grind passes:

• •**Salt and pepper (fine exposure)**: the matrix of the concrete is polished; only the smallest aggregate particles are exposed at the surface. The lightest and most consistent appearance — the least variation. Easiest to achieve consistently
• •**Medium exposure**: grinding removes approximately 1–3mm of the surface; larger aggregate particles are exposed. More variation in the visible stone; more visual interest
• •**Full aggregate exposure (seed concrete)**: the entire surface is ground back to reveal all aggregate — the floor looks like polished terrazzo. Requires careful specification of the aggregate type and colour in the concrete mix

The grinding process requires heavy specialist equipment (planetary grinders; variable-speed diamond tooling) and is done wet (using water as a lubricant and to manage concrete dust). The polished surface is then treated with a lithium silicate densifier (which chemically hardens the surface of the concrete) and sealed with a topical sealer (polyurethane; epoxy; acrylic; or penetrating sealer) for stain resistance.

• Key requirements for true polished concrete:
• •Minimum slab thickness: **75mm** (ideally 100–150mm for a ground floor extension slab — thicker is better for thermal mass and for aggregate exposure depth)
• •The concrete mix must be specified for polishing: target strength C25/30 minimum; water/cement ratio ≤0.5; aggregate selected for the desired exposed finish (if aggregate exposure is required — the slab designer must specify the aggregate type, size, and density at the mix stage — you cannot change aggregate type after pouring)
• •The slab surface must be struck to a **flatness tolerance of SR1 (super-smooth)** or **SR2** — meaning no high spots greater than 3–5mm in 2m. A poorly struck slab produces an uneven polished surface with visible grind lines
• •True polished concrete cannot be applied as a coating over an existing floor — it IS the floor. If you want true polished concrete in a new extension, it must be planned from the structural stage

*2. Concrete screed with a polished or sealed finish*:

A concrete screed (typically 50–75mm of sand-cement or proprietary floor screed, applied over a structural concrete slab or insulation layer) can also be ground and polished using the same diamond tooling. This is a common specification in London extensions where the structural slab is a standard C20/25 power-float construction and the final finish level is achieved by a screed layer that is purpose-mixed for polishing.

*Advantages over true polished concrete slab*: the screed can be applied over any structural base (including timber beam-and-block; existing concrete slab in a renovation project; or a standard extension slab). The screed is relatively thin (50–75mm) and can be specified independently of the structural slab design. A polished screed is less expensive than polishing a thicker slab.

*Disadvantages*: the screed has significantly less thermal mass than a structural concrete slab (less heat storage for UFH); a 50mm screed has limited aggregate depth so full aggregate exposure is not achievable; and screed-to-slab bond failure (where the screed debonds from the substrate) is more common than failure in a monolithic slab, particularly over in-slab UFH pipe or heating mat installations.

*3. Microcement (or 'micro concrete')*:

Microcement is NOT concrete — it is a polymer-modified thin-coat cement system (2–5mm total thickness) applied as a decorative finish over an existing substrate. Common brands: Topciment; Leroy Merlin; Artis Grout; Concrete Collective; Diasen. Microcement provides the visual appearance of polished concrete at dramatically lower thickness — it can be applied over tiles; existing concrete; plywood subfloor; or any substrate that is flat, stable, and bonded.

*Advantages*: can be retrofitted over any existing stable floor without the need for structural works; very thin (2–5mm adds negligible height to existing floor levels, avoiding complications at door thresholds and stairs); wide range of colours and finishes; rapid application (typically 2–3 days for a floor application including basecoat, top coats, and sealing); lower cost than true polished concrete in existing renovation scenarios.

• *Disadvantages and limitations*:
• •Microcement is NOT true polished concrete — it is a coating; it cannot be reground and polished like a structural slab
• •Microcement is significantly less durable than polished concrete under abrasion and point loading — it scratches relatively easily and requires periodic resealing
• •Microcement is only as good as the substrate beneath it — any movement, crack, or deflection in the substrate telegraphs through the thin coating. Microcement over timber floors (subject to seasonal movement) almost always cracks at the joist centres within 1–2 years
• •The finish range of microcement is narrower than polished concrete — it cannot expose aggregate in the way a structural slab can
• •Many clients who ask for 'polished concrete' and are offered microcement as an alternative do not fully understand the difference — ensure the specification clearly states which product is being supplied

*4. Large-format concrete-effect porcelain tiles*:

Large-format rectified porcelain tiles with a concrete-effect surface finish (manufacturers: Fioranese; Atlas Concorde; Ragno; Abk; Mirage) are sometimes marketed as 'concrete floors'. They are not concrete at all — they are a ceramic/porcelain product that mimics the visual effect of concrete. They are included here for completeness:

*Advantages*: no cracking risk; consistent appearance; wide range of sizes (60×60cm to 120×240cm large slabs); grout joints can be minimised with rectified tiles and 1mm joint specification; standard maintenance; can be heated with standard underfloor heating *Disadvantages*: grout joints (however fine) distinguish them from seamless polished concrete; the tile is thinner (6–12mm) than polished concrete and has less thermal mass; the visual quality does not match the natural aggregate variation and depth of true polished concrete for most clients who want that effect

**Underfloor heating compatibility — the critical specification issue**:

Polished concrete and underfloor heating (UFH) is an excellent combination — the high thermal mass of the concrete slab stores heat from the UFH water circuit and releases it slowly and evenly. However, the specification must be coordinated from the design stage:

• •UFH pipe or mat must be embedded in the structural slab or screed BEFORE the slab is poured or the screed is laid — it cannot be retrofitted after the fact into polished concrete
• •The concrete or screed supplier must be informed that UFH is embedded — certain admixtures and mix designs are more compatible with the thermal cycling stress that UFH imposes
• •The thermal mass of a polished concrete slab means the floor takes longer to heat up after a cold period than a thinner screed or a suspended timber floor with UFH — typically 6–12 hours to reach comfort temperature from cold. This requires controls programming for a long lead time
• •The slab must be allowed to cure fully before grinding begins (minimum 28 days; ideally 60–90 days for a polished concrete slab with embedded UFH — to allow all shrinkage to occur before the surface is finished); the UFH should be run through a commissioning protocol (gradually ramped up from 20°C to full operating temperature over 14 days) before the grinding begins, to drive out residual moisture
• •Cracking: concrete shrinks as it cures. A polished concrete slab in a London extension will typically develop hairline shrinkage cracks at 3–6m centres if not controlled with saw-cut joints. Joints must be agreed before pouring and incorporated into the design — typically along the line of structural columns or door openings. Random hairline cracks that appear through the polished surface are a natural characteristic of concrete and not a defect; this should be agreed with the client at specification stage

**Stage-by-stage process for a polished concrete floor in a London rear extension**:

*Stage 1 — Substructure and slab design*:

For a new rear extension in London, the ground floor slab is typically a reinforced concrete slab at ground level. The standard extension slab specification where a polished concrete floor is intended:

• •Slab thickness: 150mm (100mm minimum; 150mm for better thermal mass with UFH)
• •Concrete: C25/30 mix; OPC (Ordinary Portland Cement); water/cement ratio ≤0.5; if aggregate exposure is required, specify the aggregate type and colour at this stage (natural river gravel; 10mm or 20mm aggregate; or decorative aggregate such as crushed glass; metallic chip; coloured stone)
• •Reinforcement: A393 mesh (8mm bars at 200×200mm centres) for a standard domestic ground floor slab
• •Damp-proof membrane (DPM): 1200-gauge polythene DPM under the slab, lapped up to DPC level; or a tanking system where high groundwater risk applies
• •Insulation: 100–150mm of EPS (expanded polystyrene) or PIR rigid insulation below the slab, above the DPM — required for Part L compliance in a new extension (target total floor U-value ≤0.18 W/m²K for a new extension)
• •UFH pipe (if specified): 16mm MLCP pipe laid at 150–200mm centres in a serpentine pattern, clipped to the A393 mesh before concrete is poured. UFH manifold location coordinated before pour
• •Strike finish: power float to SR2 flatness (3mm tolerance over 2m) or SR1 (1.5mm tolerance over 2m) if the intention is to grind and polish without a screed overlay

*Stage 2 — Curing and drying*:

• The slab must cure for a minimum of 28 days before any finishing. During curing:
• •Prevent rapid moisture loss from the surface (cover with polythene sheeting for the first 7 days; prevent direct sun and wind drying)
• •Monitor for shrinkage cracks — saw-cut control joints at pre-agreed positions at day 7–10 (after the slab has gained sufficient strength to resist sawing but before peak shrinkage) to control crack location
• •If UFH is embedded, commission per heating protocol (14-day graduated temperature ramp from 20°C to max temperature)
• •Wait minimum 28 days before grinding begins; ideally 60 days for a UFH slab

*Stage 3 — Coarse grinding (aggregate exposure)*:

• Grinding begins with the coarsest diamond tooling (typically 30–50 grit metal bond diamond segments) to:
• •Remove the surface laitance (the weak upper layer of cement paste that rises during power floating)
• •Achieve the required aggregate exposure level (salt-and-pepper; medium; full exposure)

This stage is wet — the grinder uses water continuously; the slurry must be vacuumed and removed from the surface continuously to prevent recontamination of the surface

*Stage 4 — Progressive grinding to honed finish*:

Grinding proceeds through progressively finer grits: 30 → 50 → 100 → 200 → 400 grit, with each pass removing the scratch marks from the previous pass. By 400 grit, the floor has a 'honed' appearance (flat; consistent; matte-to-sheen finish) but is not yet polished.

*Stage 5 — Densifier application*:

A lithium silicate densifier (Consolideck LS; Retro-Plate; Husqvarna HiPERFLOOR densifier) is applied to the honed surface, left to react for 15–30 minutes, and then ground in. The densifier reacts with the calcium hydroxide in the concrete matrix to form calcium silicate hydrate crystals, which fill the surface pores and increase the surface hardness (Mohs hardness increases from approximately 4 to 7–8 after densification). This is the step that enables true polishing — a non-densified surface cannot be polished to a high sheen because the pores remain open and the surface is too soft

*Stage 6 — Fine grinding and polishing*:

• After densification, grinding continues through: 400 → 800 → 1500 → 3000 grit resin-bond diamond pads. By 800 grit the floor has a satin sheen; by 1500 grit it has a strong reflection; by 3000 grit it has a high-gloss mirror finish. The target level of polish is agreed at specification stage:
• •**Matte/honed**: stop at 400 grit (no sheen; maximum slip resistance; industrial or understated aesthetic)
• •**Satin**: stop at 800–1500 grit (moderate reflection; most popular for residential kitchens — warm and attractive without being slippery)
• •**High gloss**: 3000 grit (strong reflection; more formal; requires more maintenance to prevent scratching)

*Stage 7 — Sealing*:

• The polished surface is sealed to improve stain resistance and ease of cleaning:
• •**Penetrating sealer** (silane/siloxane/impregnating sealer): penetrates the concrete surface; does not alter sheen level; most durable for maintenance; allows the concrete to 'breathe'. Standard for most residential floors
• •**Topical sealer** (polyurethane; epoxy; acrylic): applied as a surface film; increases gloss level; provides harder surface protection; requires periodic recoating (typically every 3–5 years in a high-traffic kitchen environment)

**Common problems with polished concrete in London residential projects**:

*1. Slab too thin*: polished concrete requires a minimum 75mm slab (100mm is strongly recommended). A slab less than 75mm cannot be adequately ground without the risk of breaking through to the DPM or insulation below. Extension slabs are sometimes poured at 100mm but left short — check the slab depth before specifying polishing

*2. Wrong aggregate or poor concrete mix*: if aggregate exposure is required but the concrete was mixed with inadequate aggregate volume or the wrong aggregate size, the ground surface will show large areas of cement paste with very few exposed stones — a poor appearance that cannot be corrected without overlaying

*3. Slab flatness inadequate*: a slab with poor strike finish (large high/low spots) produces a polished surface with visible grind marks where the grinder bridged low spots. The pre-polish flatness check should be done before committing to the finish

*4. Efflorescence and moisture-related staining*: if the DPM under the slab is inadequate or bridged at edges, groundwater vapour migrating upward through the slab can create efflorescence (white crystalline deposits) under or through the sealer. This is almost impossible to remedy without full slab replacement. Always specify a high-quality DPM and ensure the slab edges are sealed to the damp-proof course

*5. Cracking*: shrinkage cracks are normal and expected — but random cracks through the polished surface are more visually prominent than in an unpolished slab. Saw-cut control joints must be designed and agreed before pouring

**2025 cost guide for polished concrete floors in London**:

Prices vary significantly based on aggregate exposure level, slab condition, area size, accessibility, and the finish level specified. All prices below assume a new extension in London with good access and a standard slab specification.

*True polished concrete (grind and polish of in-situ slab — new extension)*:

| Finish level | Slab area | Supply and install (specialist contractor; excludes slab pour) | Total inc. slab pour (new extension) | |---|---|---|---| | Salt and pepper; satin (800 grit) | 15–25 m² | £4,500–£8,000 | £8,000–£14,000 | | Medium exposure; satin (800–1500 grit) | 15–25 m² | £6,000–£10,000 | £9,500–£16,000 | | Full aggregate exposure; high gloss (3000 grit) | 15–25 m² | £8,000–£14,000 | £12,000–£20,000 | | Large area (40–60 m²) discount — salt and pepper; satin | 40–60 m² | £10,000–£18,000 | £16,000–£28,000 |

Note: the slab pour component (concrete; reinforcement; insulation; DPM; power float; UFH if included) is typically £3,000–£5,500 for a 15–25m² ground floor extension slab, or approximately £150–£250/m².

*Concrete screed with honed/polished finish (over existing structural slab)*:

| Finish level | Area | Cost per m² (supply and install) | |---|---|---| | Sand-cement screed (65mm) + honed (400 grit) | Per m² | £120–£180/m² | | Proprietary pumpable screed (60mm) + satin polish (800 grit) | Per m² | £150–£220/m² | | Specialised flowing screed + full polish (1500 grit) | Per m² | £180–£280/m² |

*Microcement finish (retrofit over existing floor)*:

| Application | Area | Cost per m² | |---|---|---| | Microcement over existing tiled or concrete floor (good condition) | Per m² | £80–£150/m² | | Microcement over existing floor (requires preparation/levelling) | Per m² | £120–£200/m² | | Premium microcement multi-colour or bespoke finish | Per m² | £180–£280/m² |

*Cost comparison summary for a 25m² London extension kitchen-diner floor*:

| Option | Typical total cost | Notes | |---|---|---| | Concrete-effect large format porcelain tile (600×600mm) | £3,500–£7,000 | Includes adhesive, grout, underfloor preparation | | Microcement retrofit | £3,000–£6,000 | Over existing stable floor | | Polished screed (new extension) | £4,500–£7,500 | Includes screed, UFH if required, polish | | True polished concrete slab (salt-and-pepper) | £8,000–£14,000 | Full slab + polish + seal | | True polished concrete slab (full aggregate exposure; high gloss) | £12,000–£20,000 | Premium aggregate; specialist grinding |

**Specification recommendations for London clients**:

1. **Agree aggregate specification in writing before the slab is poured** — the aggregate type, size, and density in the concrete mix determines the final appearance of a full-aggregate-exposure floor. Once the slab is poured, the aggregate is fixed. Use a concrete sample pour if the aggregate exposure appearance is critical

2. **Coordinate UFH with polished concrete from day one** — the UFH pipe layout, insulation specification, slab thickness, and curing protocol must all be designed as a single integrated package. Retrofitting UFH into an existing polished concrete slab is not practical

3. **Specify flatness tolerance SR2 or SR1 at the contract for the concrete contractor** — this is what you will pay the specialist polisher to work with; a poor-flatness slab significantly increases the cost of achieving a consistent finish

4. **Allow adequate curing time** — rushing to begin grinding before 28 days (or 60 days with UFH) produces a weaker surface that is more likely to scratch and discolour. Plan the construction programme to allow adequate curing before the specialist polisher begins work

5. **Microcement is a legitimate and cost-effective alternative where true polished concrete is not achievable** — in a renovation project where the existing substrate is a stable concrete slab, microcement delivers a credible polished concrete appearance at significantly lower cost. Brief the client honestly on the difference in durability and maintenance requirements

6. **Maintenance** — polished concrete floors should be cleaned with a pH-neutral cleaner (not acidic cleaners; not bleach; not vinegar — all of which etch the sealer and concrete surface). The sealer should be inspected annually and reapplied where worn (typically every 3–5 years in a kitchen environment). Polish is not permanent — it can be re-done by a specialist after 10–15 years if the surface has dulled