Order custom casement window frames.

Investing in bespoke frames allows you to specify everything from timber species and ironmongery to high-performance glazing. This guide outlines the genuine costs, timelines, and regulatory requirements for commissioning custom casement window frames in the UK.

What Does the Work Involve?

• Site Survey: Precise measurement of the masonry opening, checking for squareness and structural integrity of the lintel.
• Design Specification: Selecting the opening configuration (top-hung or side-hung), timber type (e.g., Accoya, Oak, or Softwood), and glazing bar styles.
• Workshop Fabrication: Machining the timber profiles, creating mortise and tenon joints, and assembling the frame and sashes.
• Preservation and Coating: Applying anti-fungal treatments followed by factory-applied primers, stains, or microporous paints.
• Glazing and Sealing: Fitting the double or triple-glazed units with appropriate spacers, gaskets, and internal or external beads.
• Hardware Fitting: Installing "ironmongery" including friction stays, espagnolette locking systems, and handles.
• Installation: Removing the old frame, fixing the new unit, and applying perimeter seals and internal architraves.

Typical Costs

The following prices reflect realistic 2025/26 UK averages including VAT. Prices vary significantly based on material choice, with hardwoods and engineered timbers commanding a premium over standard uPVC or softwood.

Material choice is the primary cost driver; for instance, Accoya timber can cost 30-40% more than softwood but offers a 50-year anti-rot guarantee. Size, glazing specifications (such as acoustic or solar-control glass), and the complexity of the opening also influence the final quote.

How Long Does It Take?

• Survey and Quoting: 1–3 days from initial enquiry to receiving a formal proposal.
• Workshop Lead Times: 4–12 weeks depending on the joinery’s current backlog and material availability.
• Fabrication: 1–2 days of actual workshop time per frame for machining and assembly.
• Painting/Finishing: 3–5 days to allow for multiple coats and drying times in a temperature-controlled environment.
• On-Site Installation: 2–4 hours per window, typically completing a whole-house replacement in 2–3 days.

DIY or Professional?

While a competent DIYer can technically install a window, it is rarely recommended for custom frames. Professional installation is required to ensure the unit is perfectly level, weather-tight, and compliant with UK thermal efficiency standards.

Replacing windows is a "controlled service" under UK Building Regulations. Using a registered professional (FENSA/Certass) saves you the cost and hassle of an independent Building Control inspection.

Choosing the Right Tradesperson

• Check Accreditations: Ensure they are members of FENSA, Certass, or the British Woodworking Federation (BWF).
• View Previous Work: Ask to see photos of recent "in-situ" custom installs, specifically focusing on the neatness of the perimeter pointing.
• Verify Guarantees: Look for insurance-backed guarantees (IBGs) that protect your deposit and warranty.
• Question: "Are your frames internally glazed to prevent glass removal from the outside?"
• Question: "Does the quote include the disposal of my old frames and glass?"
• Question: "What is the U-value of the completed unit, and does it meet current Part L requirements?"

UK Regulations

• Part L (Thermal): New windows must achieve a U-value of 1.4 W/m²K or better (or a Window Energy Rating of B or higher).
• Part F (Ventilation): Most replacement windows must include "trickle vents" to ensure adequate background ventilation and prevent mould.
• Part K (Protection from Falling): Toughened safety glass is mandatory for windows within 800mm of the floor or in specific "critical locations."
• Part B (Fire Safety): Certain windows must serve as emergency "fire escape" exits, requiring specific opening sizes and hinges.
• FENSA/Certass Certificate: You must receive this document after installation; it is essential when selling your home.

Common Problems

• Incorrect Measurements: If a custom frame is even 10mm too large, it may require structural alterations to the masonry, significantly increasing costs.
• Poor Drainage: Custom timber frames must have correctly machined "weep holes" or drainage channels to prevent water sitting and rotting the wood.
• Inadequate Finishing: If timber frames are not sealed on all six sides (including the bottom edge) before installation, they will absorb moisture and warp.
• Cold Bridging: Poorly fitted frames without proper expanding foam or thermal breaks can lead to condensation and dampness around the internal reveals.

Frequently Asked Questions