How Building Contractors in London Handle Single Storey Extensions From Start to Finish

Ever wondered why one extension takes eleven weeks and another, roughly the same size, drags on for six months? It’s rarely bad luck. It usually comes down to whether the contractor actually followed a proper sequence, or was making decisions on the fly as they went. A homeowner in Balham told me this after her own extension wrapped up — she said the thing that surprised her most wasn’t the mess or the noise, it was how predictable the whole thing turned out to be once she understood what was supposed to happen at each stage.

So here’s what actually happens, in order, when proper building contractors in London take on a single storey extension — and why knowing this sequence yourself is one of the best ways to spot whether a contractor knows what they’re doing.

Before Anyone Picks Up a Tool

The stage that matters most on any extension happens before construction even starts, and it’s the one homeowners are most tempted to rush through. A proper site visit looks at your garden, your boundaries, existing drainage, and anything specific to your borough that might affect what’s achievable. This is also where a contractor should tell you plainly whether your extension needs full planning permission or falls under permitted development — get that wrong early and you’re looking at delays nobody signed up for.

Design comes next, and for single storey extensions specifically, it’s less about chasing anything dramatic and more about getting the practical details right — how the new space connects to the existing kitchen, where the roof lantern or bifold doors sit, whether the layout actually suits how you live rather than just how it looks in a render. Structural calculations should happen alongside this, not after, so what gets submitted for planning is something that can actually be built at the price you’ve been quoted.

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Getting the Paperwork Sorted

Once designs are finalised, planning applications go in where needed, along with building regulations submissions. This is usually the slowest part of the whole process, mostly because it’s largely out of anyone’s hands while the council reviews it. A good contractor manages this for you, chasing any queries from the planning department and giving you a realistic timeline rather than an optimistic one.

Party wall agreements, if your extension touches a shared boundary, get sorted in this window too. It’s a step homeowners managing things themselves often leave too late, and experienced building contractors in London know to start it early, since neighbours don’t always reply quickly.

Groundworks: The Unglamorous Start

Once approvals are through, the physical build starts with groundworks — digging footings, laying foundations, diverting drainage where needed. Nobody finds this stage exciting to watch, but it’s exactly where problems with the ground itself surface, if there are any. Older London properties occasionally hide surprises down there, from long-forgotten drains to soil that needs extra work. A contractor who’s thorough here catches issues while they’re still cheap to sort out, not once the walls are already up.

The Structural Shell Goes Up

With foundations down, the structure starts taking shape — brickwork or blockwork, structural steel where you’re opening into the existing house, and the roof itself, whether that’s flat, pitched, or built around a lantern. This is usually the point where the extension starts feeling real, because for the first time you can actually see the shape of the room you’re getting.

Windows and doors, including any bifold or sliding sets, typically go in near the end of this stage — and that’s the moment the building becomes weathertight, which matters, because everything after this can carry on regardless of what’s happening outside.

First Fix: Everything That Disappears Behind the Walls

This is the stage nobody photographs, and yet it’s the one I’d argue matters most. Pipework, electrical cabling, any underfloor heating loops — all of it goes in now, before a single wall gets plastered over. Once that plaster’s on and dry, that’s it. Anything wrong in there means chasing back into a finished wall to fix it, which is exactly the kind of job nobody wants halfway through a project that’s supposed to be nearly done.

It’s also, quietly, when insulation gets packed into the walls, floor and roof — the part of the build that decides whether your new room is genuinely comfortable to sit in come January, or one you end up avoiding because it never quite warms up.

Second Fix and Finishes

Once the plaster’s dry, things start moving fast, and this is usually the point where the project stops feeling like a building site and starts feeling like a room. Sockets go in, light fittings get wired up, radiators or heating controls are connected, and if there’s a new kitchen involved, this is when it actually starts taking shape.

It’s also the stage where homeowners tend to get properly hands-on. Structural decisions are behind you by now — this bit’s about taste. Flooring, tiling, paint colours, the small landscaping touches that tie the new extension into the rest of the garden. It’s the fun part, honestly, after weeks of watching walls go up.

Snagging and Handover

A job worth doing properly doesn’t just stop the moment the last wall’s painted. Snagging means someone actually walking the finished space with a proper eye — checking doors close the way they should, finishes are even, nothing’s been left half-done in the rush to wrap up. Any decent contractor treats this as its own stage, not something squeezed in on a Friday afternoon before they’re off to the next job.

Why Knowing This Sequence Actually Helps You

This isn’t just background information for its own sake. Knowing the order things happen in means you know what to expect, when, and roughly how much noise and disruption belongs to each stage. If a contractor can’t walk you through this clearly before work starts, or keeps giving vague answers about “we’ll see how it goes,” that’s worth paying attention to.

How We Run Our Projects

At London Design & Build, every single storey extension follows this same clear sequence, with a dedicated project manager keeping you informed at each stage rather than leaving you to guess based on the noise coming from your garden. Planning, building regulations, and party wall matters are all handled as part of the service, and pricing is agreed upfront and held to throughout.

In Conclusion

Knowing how a project actually unfolds, stage by stage, is one of the clearest ways to judge whether a contractor genuinely knows the job. London Design & Build offer a free site visit to talk you through exactly how your single storey extension would progress from first design to final handover. Get in touch and we’ll give you a clear, honest picture of what to expect for your home.