Should You Re-Roof When Having an Extension Built?

Why the Timing of Your Extension Build Matters for Your Roof

When a builder quotes you for a rear extension or loft conversion, the roof above your existing home rarely gets mentioned. It sits there quietly while everyone argues about kitchen layouts and bi-fold doors. But if your main roof is ageing, an extension project is almost always the most cost-effective moment to address it — and missing that window can be an expensive mistake.

In Mildenhall and the surrounding villages, we see a mix of 1960s and 1970s semi-detached houses, older terraced properties and some newer builds. Many of the older homes still carry their original concrete interlocking tiles or even ageing clay plain tiles. If your roof is 25 to 35 years old and you're about to invest £30,000 or more in an extension, it's worth having a roofer assess the main roof at the same time as the extension roof is being planned.

The Practical Case for Combining the Work

Scaffolding is the single biggest driver of roofing costs on most domestic jobs. Erecting and striking a full scaffold can cost anywhere from £800 to £2,500 depending on the size and access difficulty of your property. When scaffolding is already in place for an extension, extending it to cover the main roof adds a fraction of that cost — often just a few hundred pounds extra.

Labour works the same way. A roofing crew already mobilised on site, with skips booked and materials being delivered, can strip and re-roof a standard three-bedroom semi in two to three days. Starting that job separately means fresh mobilisation costs, another skip hire, and the disruption of a second round of scaffolding and access arrangements.

If your existing roof needs attention in the next five years anyway, doing it now alongside your extension is almost always cheaper overall. A full roof replacement on a typical Mildenhall semi costs roughly £5,000 to £9,000 depending on tile type and roof complexity — combining it with an extension build can save you £1,000 to £2,000 compared to tackling it independently later.

When You Should Re-Roof During an Extension Build

Not every homeowner needs to re-roof just because they're building an extension. But there are clear indicators that it makes sense to do so:

• Your roof is over 20–25 years old and has never been fully overhauled. Concrete tiles start losing their surface integrity and felts deteriorate around this age.
• You've had repeated small repairs — if we've patched your roof more than twice in five years, the underlying structure is likely telling you something. See our roof repairs page for context on when repairs stop making sense.
• Your extension will join or butt against the existing roof, requiring lead flashings, valley work or new tiling to marry the two together. Doing this neatly is far easier when you're reroofing the whole slope rather than trying to integrate new materials into a tired old roof.
• You're adding a flat-roofed extension. If the new extension has a flat or low-pitch section, ensuring your existing pitched roof is in good condition above it prevents moisture problems at the junction. Our flat roofing team handles these junctions regularly across Mildenhall and out to Lakenheath and Red Lodge.
• Your felt or battens are failing. In the Breckland area, the dry easterly winds and sharp temperature swings between winter and summer put real stress on roofing felt. Once felt starts to crack and sag between rafters, water management on the roof breaks down regardless of how good your tiles look from the ground.

Planning Permission and Building Regulations

Any extension will require building regulations approval, and the building inspector will look at how the new roofline integrates with the existing structure. In some cases — particularly where the extension raises the eaves line or changes the ridge height — work on the existing roof may be required to satisfy regulations rather than just being optional. You can check whether your project triggers a full planning application at GOV.UK's planning guidance pages.

If you're using a roofing contractor for any part of the work, it's also worth checking they're registered with the National Federation of Roofing Contractors (NFRC), which gives you a baseline assurance of competence and the ability to raise complaints if needed.

What to Ask Your Builder Before Work Starts

Builders often subcontract the roofing element, which can mean the roofer turns up without having seen the existing roof. Ask your builder explicitly who is assessing the main roof and what condition it's in before scaffolding goes up. If there's no clear answer, bring in your own roofer for an independent inspection first.

Questions worth asking: How will the new extension roof tie into the existing slope? What flashings and junctions are planned? If the existing felt is damaged when the scaffold goes up, is there a plan to address it? Getting these answers upfront avoids nasty surprises halfway through a build.

We regularly work alongside local builders across Mildenhall, Beck Row and the surrounding area, and we're happy to attend site before a build starts to give an honest assessment of the existing roof's condition — with no obligation to use us for the work.

Get a Free Roof Survey Before Your Extension Starts

If you're planning an extension and want an honest opinion on whether your existing roof should be tackled at the same time, contact Mildenhall Roofers for a free local roof survey. We'll inspect your existing roof, give you a written report and a fixed price so you can make an informed decision before the scaffolding goes up — not after.

More advice