How Much Does a Flat Roof for an Extension Really Cost in Sussex?

It’s the question I get asked every week from homeowners in Brighton, Hove, and Worthing: “Jan, what’s the real cost for a flat roof on my new extension?” You’ve got the builder sorted for the walls, but the roof is a specialist job, and you want to know what to budget.

Let’s get straight to it. If you’re looking for a quick, ballpark figure, you should budget somewhere between £150 to £250+ per square metre (£/m²) for a professionally installed, high-performance warm deck flat roof.

But that’s a huge range, right? The truth is, any roofer who gives you a fixed price per metre over the phone without seeing the job is guessing. The final cost depends on a lot more than just the size. Let me break down exactly what you’re paying for.

What Actually Goes Into The Cost? A Roofer’s Breakdown

When I put together a fixed-price quote, I’m not just pulling a number out of thin air. I’m calculating the cost of every single component and the labour involved. Here’s what’s on my list:

1. The Roofing System Itself

This is the most obvious part. The waterproof membrane you choose has a big impact. Your main options are:

  • EPDM (Rubber): Excellent choice. It’s durable, has a long lifespan (often 40+ years), and is great for handling the Sussex weather. It’s one of my preferred systems.
  • GRP (Fibreglass): Another top-tier option. It creates a seamless, hard-wearing finish that’s perfect if you’re planning a balcony or roof terrace. Very tough.
  • High-Performance Felt: Modern torch-on felt is a world away from the stuff used 30 years ago. It’s a solid, reliable choice, but often has a slightly shorter lifespan than EPDM or GRP.

For most residential projects, I steer people towards modern flat roofing systems like EPDM or GRP for their longevity and reliability.

2. The Roof Structure & Insulation (Warm vs. Cold Roof)

This is crucial and non-negotiable for a new extension. You’ll need a ‘warm roof’ to meet building regulations. This means a solid timber deck (usually 18mm OSB board), a vapour barrier, a thick layer of rigid insulation (like 120mm PIR board), and then the waterproof membrane on top. This setup prevents condensation and drastically cuts your heating bills. A cheap ‘cold roof’ (with insulation between the joists) is a recipe for damp and mould down the line.

3. The ‘Extras’ That Aren’t Really Extra

This is where cheap quotes often fall apart. A roof isn’t just a flat sheet. It needs edges, drainage, and connections to the main house. These details are critical for a leak-free finish.

  • Edge Trims & Drips: Professionally finished uPVC or metal trims that direct water away from the brickwork.
  • Flashing: Usually leadwork, which seals the join between your new extension roof and the existing house wall. Skimping on this is asking for trouble.
  • Fascias & Soffits: Your new roof will need a new edge. I always recommend fitting new uPVC fascias and soffits for a maintenance-free finish.
  • Guttering: The water has to go somewhere! You’ll need proper guttering and a downpipe connected to your drains.

4. Skylights and Roof Lanterns

Want to flood your new space with light? A skylight or a large roof lantern is a fantastic addition, but it adds significant cost and complexity. It’s not just the price of the unit itself; it involves creating a timber upstand, waterproofing it meticulously, and ensuring it’s all perfectly sealed. This requires skill and time, which will be reflected in the quote.

Why a ‘Too Good to be True’ Quote is a Major Red Flag

I’ve seen it all across Brighton and Worthing. A homeowner gets a quote that’s £1,000 cheaper than everyone else’s. Six months later, I’m there fixing the leaks. The cheap quote often means they’re cutting corners: using thin insulation (or none at all), cheap materials that will fail in five years, skipping essential flashing, or just doing a messy job that looks terrible and won’t last.

My quotes are fixed-price for a reason. They include everything needed to do the job right, once. That includes proper materials, waste disposal, scaffolding if needed, and my time on your roof. You can even log into my client portal and see daily photo updates of the work, so you know exactly what you’re paying for.

So, How Do You Get an Accurate Price?

Stop guessing. The only way to get a true cost for the flat roof on your new rear extension is to have a roofer look at your plans and the site. I can assess the exact size, complexity, access, and detailing required.

I don’t do ballpark figures over the phone because it’s not fair to you. Instead, I provide a detailed, itemised, fixed-price quote after a proper survey. If you’re in the Brighton, Hove, or Worthing area and want an honest, no-nonsense price for your extension roof, the best first step is to book a proper roof inspection. I’ll take a look and give you a figure you can actually rely on.