The extension process: why projects take twice as long as expected (and how to plan for reality)

Chris Brace

Founder, Planning to Build

17th August 2025

Whether you're just exploring the idea of an extension or you've already committed to the project, there's one thing every homeowner discovers: the timeline you've initially got in mind rarely matches reality. The architect promises 6 months, the builder nods along, and you're already imagining Christmas dinner in your new kitchen. Then month 8 arrives, and you're still staring at exposed brickwork where your dream space should be.

We know that sinking feeling when timelines slip and slip again. A lot of the homeowners we speak to have the same story: "They said 6 months, but we're coming up on 18 months and still not finished." Your frustration isn't unique: it's the predictable result of how extension projects actually unfold when crucial information gets missed in the handoffs between professionals.

What actually happens when you build an extension?

If you're new to home extensions, here's what the journey typically looks like: You'll work with an architect to design your space, apply for planning permission from your local council, hire a structural engineer if walls need moving, get a builder to construct it and possibly deal with party wall agreements if you're close to neighbours.

Most extensions take between 12-18 months from first sketch to finished space - but here's where it gets tricky. Each professional handles their piece as efficiently as they can, but extension problems happen in the gaps between them, when crucial information doesn't get passed along properly.

Whether you're just starting to explore what's possible or you're already deep in the planning process, understanding where projects typically go wrong will help you ask the right questions and avoid the predictable pitfalls.

The problem: when professionals miss things, projects spiral

Most homeowners do exactly what they should do: they hire experienced professionals and trust them to guide the process. Architects, builders, and engineers are genuinely trying to help, and most are skilled at what they do.

But extension projects involve multiple specialists, each focused on their own piece of the puzzle. Timeline disasters happen when crucial information gets missed in the handoffs:

The architect designs your extension and handles planning applications, but focuses on aesthetics and space planning rather than construction details. They may not spot structural complications that could add months to your timeline.

The builder prices and constructs the work based on plans and specifications, but quotes before all the details are known. They can't anticipate planning delays or party wall issues that haven't been flagged yet.

The structural engineer calculates how to safely remove walls and support loads, but typically gets involved after planning is underway. They solve technical challenges but aren't usually coordinating timeline with other aspects.

Party wall surveyors handle neighbour agreements for work near boundaries - a process that can take months but often isn't started early enough in the planning sequence.

Everyone assumes someone else is tracking the big picture and keeping all these moving parts coordinated on realistic timelines.

When any piece of crucial information gets missed, about your property, the local planning process, or coordination constraints — those small gaps compound into major timeline overruns.

How these timeline problems impact your project

Even when professionals coordinate perfectly, three other factors frequently derail timelines, and understanding these helps you make decisions that keep everyone's schedule intact.

Factor 1: Supply chain delays - when materials become the bottleneck

That beautiful set of bifold doors you selected? They might take 14 weeks to manufacture and deliver. Bespoke windows, specialist tiles, and custom kitchen units all have lead times that many homeowners don't realise need to be built into the construction sequence from day one.

When homeowners manage procurement themselves to save money, they often discover too late that their "quick order" for upgraded materials pushes completion back by months. Your builder can't install what hasn't arrived, and rescheduling trades around delayed deliveries creates a domino effect through their entire schedule.

Factor 2: Scope changes - why "small" adjustments aren't small

You see the space taking shape and suddenly realise you want to move that socket or upgrade to underfloor heating. Each change seems minor in isolation, but requires coordination across multiple trades working to their own timelines.

Moving one electrical socket might seem simple, but it affects the electrician's schedule, the plasterer who needs to patch the wall, and the decorator who planned to finish that area next week. What feels like a quick adjustment can push your project completion back weeks because it disrupts the carefully orchestrated sequence of work.

Factor 3: Builder scheduling reality - the invisible coordination challenge

Here's what most homeowners don't see: your builder isn't just working on your project. They're coordinating multiple jobs with different tradesmen: electricians, plumbers, plasterers, and tilers, all working to their own packed schedules across different sites.

When your project shifts by even a week due to planning delays or material issues, your builder faces a complex puzzle: how to realign everyone's availability when the electrician is now committed to another job, the plasterer is booked solid for the next month, and the kitchen fitters have moved on to their next installation.

This is why the best builders build buffer time into their schedules and why understanding these constraints helps you make decisions that keep the whole ecosystem moving smoothly. When you appreciate the coordination challenge your builder faces, you can work together to prevent the small disruptions that create major delays.

What you can do about it

Here's what makes the difference: when homeowners understand their project's potential complications early on, they can help ensure nothing gets missed in the professional handoffs.

Think of it like this: if you already knew about potential foundation issues with your neighbour's property, you could make sure your architect and structural engineer coordinated on that from the start. If you understood realistic construction schedules, you could time your planning applications to align with builder availability.

Professional builders naturally think about extension projects in three phases because they understand the full journey from initial idea to finished space, and they've seen where things typically go wrong. Whether you're just starting to explore what's possible or you're already committed to building, understanding these phases helps you spot where timeline disasters typically happen:

Phase 1: research and budget (2-4 months)

Understanding what's actually possible with your specific property:

• Structural complications: Some walls are load-bearing (holding up the house) and need special steel beams to move safely. Some foundations need underpinning if you're digging deeper. These discoveries can add weeks or months to your timeline.
• Planning permission requirements: Your local council has rules about how big extensions can be, what materials to use, and whether neighbours need to approve. Some areas have extra restrictions that aren't obvious until you dig into the details.
• Party wall processes: If you're building within 6 meters of a neighbour or extending up to your boundary, you may need formal agreements that can take 2-3 months to complete.
• Design choices and construction sequence: A simple rear extension might take 6 months to build, but add a loft conversion and side return, and suddenly you're looking at 12+ months because of how the work has to be sequenced.

Phase 2: planning and design (6-12 months)

Getting all approvals coordinated:

• Planning applications with realistic timelines
• Party wall processes started early
• Structural work planned with discoveries already anticipated
• All professionals working from the same timeline expectations

Phase 3: build (6-12 months)

Construction with fewer surprises:

• Potential complications already identified and planned for
• Build sequence planned around trade availability and project coordination
• All trades coordinated around realistic milestones

When homeowners understand these phases and potential complications early, they can help their professionals stay coordinated and avoid the gaps where timeline disasters happen.