External Render for London Extensions: Silicone, Monocouche & Sand-Cement Compared

External render is the most common external finish for rear and side extensions on brick or block construction in London — particularly where the external face is not visible from the street and a contemporary smooth finish is preferred over facing brickwork.

Why Render on Extensions?

Extensions are often built in blockwork (concrete block) rather than facing brick, because the external face is within the curtilage and not visible from the public realm. Render provides a clean finish over blockwork, can be colour-matched to existing render, and is lower cost than facing brickwork.

Sand-Cement Render

The traditional render system — a scratch coat of sand-cement mortar followed by a finish coat, painted over when dry. Widely used and understood by all plasterers.

Advantages:

• Low cost
• Suitable for repairing or patching existing sand-cement render
• Can be painted in any colour

Disadvantages:

• Prone to cracking as the house moves and the render shrinks during curing
• Requires repainting every 5–8 years
• Not breathable — can trap moisture behind the surface if not properly detailed

Cost: £18–£28/m² for supply and apply (labour and materials)

Monocouche Render (Single-Coat Render)

A factory-produced, through-coloured cement-based render applied in a single coat (though the coat may be built up in passes). The colour is integral — no painting required. Common brands include Weber.pral M and K Rend Silicone TC.

Advantages:

• Through colour — scratches reveal same colour underneath, no bare patches
• No painting required for 10–15 years
• Better crack resistance than traditional sand-cement (polymer additives)
• Clean, contemporary finish

Disadvantages:

• More expensive than sand-cement
• Colour limited to range offered by the product
• Application requires experience — not all plasterers are trained in monocouche

Cost: £28–£40/m² for supply and apply

Silicone Render

A polymer-modified, silicone-enhanced through-coloured render system. Applied over a base coat (typically EWI board or direct to existing render). The most premium render system for new-build applications.

Advantages:

• Highly flexible — dramatically better crack resistance
• Water repellent — hydrophobic surface sheds water
• Breathable — allows moisture vapour to pass from inside to out
• Wide colour range
• Self-cleaning properties in some products (dirt washes off with rain)
• Very long maintenance cycle — 15–25 years

Disadvantages:

• Highest cost
• Requires specialist application

Cost: £40–£60/m² for supply and apply (direct to substrate); higher if applied over EWI board system

Typical Specification for a Rear Extension

For a standard single-storey rear extension where the render is on the external face of the rear and side walls:

• Area: typically 25–50m² depending on size
• Monocouche render is the most common specification — colour-matched to existing render on the house
• Silicone render where a premium finish or better performance is required

RCB Design & Build

RCB Design & Build specifies and applies external render systems as part of extension delivery — advising clients on the appropriate system for each project's requirements, existing finishes, and budget.