How Long Does It Take to Build a Home Extension?

A house extension is one of the most rewarding investments you can make in a property, but the question we hear more than almost any other is: how long does a house extension take? The honest answer is that it depends, but with the right information you can plan with real confidence.

Why There’s No Single Answer

Every project is different. Before we can give you a reliable timeline, we need to understand the full picture: what you’re building, where, and whether planning permission applies.

That said, the construction industry does follow fairly predictable patterns, and after more than three decades of delivering extensions across Kent and the South East, we can speak with confidence about what realistic timelines look like at every stage.

Stage One: Design and Architectural Drawings (4–8 Weeks)

Before a single brick is laid, you need a set of architectural drawings. This is where most homeowners tend to underestimate the time involved. A good architect won’t just sketch what you’ve described — they’ll carry out a measured survey of the existing property, review permitted development limitations and any local planning constraints, and produce drawings detailed enough to submit for planning or building regulations approval.

Four to eight weeks is a reasonable expectation for most residential extension designs. More complex projects can take longer. Getting this stage right matters enormously. Rushing drawings often leads to problems and additional costs further down the line.

At Hargrave Construction & Renovations, we have well-established relationships with local architects, structural engineers, and the planning teams at councils across Kent. That network means things tend to move more smoothly, and more quickly, than they might otherwise.

Stage Two: Planning Permission (8–12 Weeks, If Required)

Not all house extensions need full planning permission. In England, many residential extensions qualify under Permitted Development (PD) rights, which means you can build within defined parameters without making a formal planning application. This can save you two to three months on your overall programme.

However, PD rights come with conditions. The size, height, materials, and position of your extension all affect whether it qualifies. Properties in Conservation Areas, Areas of Outstanding Natural Beauty, or those that are listed buildings face stricter rules and often require full consent even for smaller works.

If a planning application is needed, the local planning authority has eight weeks from the date of a valid application to make a decision. In practice, this is generally met for straightforward householder applications, though some councils request an agreed extension to the period, something worth clarifying early on.

Even when you believe your extension falls within Permitted Development, it’s worth obtaining a Lawful Development Certificate from your local council. This provides formal confirmation and removes any ambiguity when you come to sell the property in future.

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Stage Three: Structural Engineering and Building Regulations (2–6 Weeks)

Separate from planning permission and often confused with it, Building Regulations approval is required for virtually all house extensions. This is the process that ensures your extension is structurally sound, thermally efficient, and safe in terms of fire protection and electrical and plumbing installations.

A structural engineer will produce calculations to support the building regulations application, covering elements such as foundation design, beam sizing, and load paths. Once submitted, a building control officer will inspect the works at key stages throughout the build. Obtaining structural calculations and submitting a building regulations application usually takes two to four weeks, though approval itself is typically confirmed at first inspection once work is underway.

Stage Four: Pre-Construction (2–4 Weeks)

Once consent is secured and drawings are finalised, there’s typically a preparation period before work can begin on site. This includes procuring materials, confirming the programme with all trades, arranging scaffolding, liaising with utility companies if services need diverting, and ensuring everything is ready for a smooth start. Projects that start without everything in place tend to experience unnecessary delays: waiting for materials, chasing decisions, or accommodating trades that aren’t available when needed. We’re rigorous about pre-start planning because it directly affects how consistently progress is made on site.

Stage Five: The Build: How Long Does Construction Actually Take?

This is the stage most homeowners are asking about, and it’s where the answer varies most. Here’s a realistic breakdown by extension type:

EXTENSION TYPE	TYPICAL BUILD TIME	KEY VARIABLES
Small rear single-storey (up to 4m)	8–10 weeks	Ground conditions, roof type, internal finishes
Larger single-storey (4m–8m)	10–16 weeks	Size, number of rooms, glazing specification
Double-storey extension	16–22 weeks	Structural complexity, roof integration
Wrap-around extension	20–28 weeks	Scale, multiple gable ends, internal reconfigurations
Side return extension	10–14 weeks	Party wall matters, access, structural opening width

Ready to Start Planning Your Extension?

We have been delivering extensions across Kent and the South East for over 30 years. We’ll visit your property, listen to what you need, and give you an honest timeline and quote.

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