Last Updated on May 6, 2025

How to Replace A Casement Window: A Step-by-Step Guide for Homeowners

Replacing a casement window is one of the most effective ways to improve your home’s energy efficiency, security, and curb appeal. Whether you’re a UK homeowner looking to upgrade draughty windows or a landlord aiming to meet EPC requirements, knowing how to replace a casement window can help you make informed decisions and avoid costly mistakes.

In this comprehensive guide, our experts at Eco Happy walk you through the entire process of replacing a casement window, from removing the old frame to installing the new unit. We’ll also highlight when it’s best to DIY and when it’s smarter to call in a FENSA-registered professional.

Why Replace Your Casement Windows?

Over time, even the most durable windows show signs of wear. Here are the key reasons to replace a casement window:

• Increased energy bills due to poor insulation
  
• Visible damage such as rot, condensation between panes, or warped frames
  
• Difficulty opening or closing the window
  
• Outdated style reducing kerb appeal
  
• Upgrading to double or triple glazing for better efficiency
  

Replacing windows is also a strategic move for landlords aiming to boost EPC ratings—a requirement under UK housing regulations.

Tools and Materials You’ll Need

Before starting, gather the following:

• New casement window unit
  
• Screwdriver and drill
  
• Crowbar or pry bar
  
• Tape measure
  
• Expanding foam
  
• Window packers
  
• Spirit level
  
• Silicone sealant
  
• Safety gloves and goggles
  

Note: Always check that your new windows meet UK building regulations, especially for thermal performance (U-values).

Step-by-Step: How to Replace a Casement Window

1. Measure the Opening Accurately

Use a tape measure to calculate the height and width of the window opening. Always measure in millimetres and check both diagonals to ensure squareness. Ordering the wrong size window is a costly mistake—accurate measurement is critical.

2. Remove the Existing Window

Removing an old casement window isn’t just about unscrewing and pulling—it requires care and precision to avoid damaging surrounding walls, reveals, or brickwork. Here’s how to do it safely and effectively:

• Open the casement fully to access the hinges and screws. If the window is painted shut or stiff from age, gently use a utility knife to break the seal first.
  
• Unscrew the hinges carefully using a drill or manual screwdriver. If your window is top-hung or has stays, you may need a second person to hold it in place as you detach it.
  
• Remove any trim or beading around the frame using a flat pry bar or a wide chisel. This step reveals the fixings that hold the frame to the wall. Take your time to avoid damaging the internal plaster or render.
  
• Pry the window frame from the wall using a crowbar, starting from one corner and working your way around. If it’s an older timber frame, expect resistance due to swelling, expansion, or rusted fixings.
  

Top Tip: Homes built before the 2000s may contain asbestos in window boards, mastics, or surrounding materials. If you suspect this, stop work immediately and consult a licensed asbestos surveyor or removal specialist. Never cut corners on health and safety.

3. Prepare the Opening

Now that the old window is out, prepping the space correctly is vital for a snug, durable fit.

• Clear out all debris, dust, and old sealant from the opening. A clean, dry surface ensures adhesives and insulation materials bond properly.
  
• Inspect the brickwork or timber surround for signs of rot, crumbling mortar, or damp. Any defects should be repaired before installing the new frame.
  
• Check the sill is level using a spirit level. If not, use non-compressible window packers or shims to adjust and create a true base for the new frame. Proper levelling prevents issues with window operation later on.
  
• Apply a damp-proof membrane or tape, if necessary, to protect the masonry from moisture transfer. This step is especially important in older homes with no cavity wall.

Failing to prepare the opening correctly can lead to cold bridging, draughts, or even structural movement post-installation.

4. Install the New Casement Window

With the opening ready, it’s time to fit your new casement window—this step must be executed with care to ensure longevity and performance.

• Place the window into the opening, positioning it on the prepared sill and aligning it with the packers. Make sure any drainage holes are facing outward.
  
• Use a spirit level to ensure the window is plumb (vertically straight) and level (horizontally aligned). Adjust using additional packers if necessary. An out-of-square window will affect sealing and opening/closing mechanisms.
  
• Secure it temporarily with packers at strategic points—top, bottom, sides—to hold it firmly in place while drilling.
  
• Drill through the pre-drilled fixing holes in the frame and into the surrounding wall using masonry or timber fixings, depending on your substrate. Space screws evenly (typically every 600mm), ensuring a solid and stable installation.
  
• Fill the perimeter gap with low-expansion expanding foam. This foam acts as an insulator and helps secure the window. Avoid overfilling, as it can bow the frame if it expands too much.
  

This stage determines the long-term performance of your window. Poor alignment or uneven fixings can cause operational issues, water ingress, or even invalidate warranties.

5. Seal and Finish

With the window securely in place, the finishing touches are just as important as the frame itself. They prevent air and moisture from getting in—and help the installation look polished.

• Once the foam cures (usually after 1–2 hours, but follow the product’s instructions), carefully trim any excess with a utility knife to ensure a neat finish.
  
• Apply high-quality silicone sealant around the external edge of the frame where it meets the wall. This forms your first line of defence against rain and wind. Inside, use decorators’ caulk if painting over, or silicone for a permanent flexible seal.
  
• Reinstall interior trim, architrave, or beading, depending on your home’s finish. If you’ve removed plaster during installation, repair and paint as needed.

When completed properly, your new casement window will be airtight, draught-free, and beautifully integrated with your property’s aesthetic.

How Much Does It Cost to Replace a Casement Window in the UK?

Prices vary depending on the size, frame material, glazing type, and installer. On average, replacing a casement window in the UK costs between £300–£750 per window. Timber frames tend to cost more than uPVC, while triple glazing comes at a premium.

Curious about the full cost breakdown? Read our detailed guide on casement window costs to get a better idea of what to expect based on your specific requirements.

But remember: labour costs, energy efficiency, and long-term savings also play a huge role. Replacing windows isn’t just a cost—it’s an investment in your home’s value and comfort.

Should You DIY or Hire a Professional?

Replacing a casement window yourself is possible if you have the right tools, time, and confidence. But for most UK homeowners and landlords, hiring a certified window installer ensures:

• Compliance with FENSA or Certass requirements
  
• Proper insulation and sealing
  
• A manufacturer’s warranty remains valid
  
• Reduced risk of damage to brickwork or plaster

At Eco Happy, our skilled team specialises in energy-efficient, regulation-compliant window replacements tailored to UK properties—whether it’s a Victorian terrace, a modern flat, or a commercial building.

Eco Happy: Your Trusted Window Replacement Experts

Choosing Eco Happy means choosing peace of mind:

• Expert guidance from survey to installation
  
• High-performance double and triple glazing options
  
• Competitive prices, transparent quotes
  
• Fully insured and certified professionals
  
• Eco-conscious solutions to cut carbon and costs

Request your free window replacement quote here and find out how we can help you transform your home today.

Conclusion

Now you know exactly how to replace a casement window—from removing the old unit to sealing the new one. Whether you’re looking to improve your home’s efficiency or modernise your property, this step-by-step guide gives you the confidence to move forward.

But why go it alone? Save time, ensure compliance, and enjoy lasting results by letting Eco Happy handle your window replacement project from start to finish.