Thinking About a Two Storey Extension? Here's What I Wish Someone Had Told Me First

 

If you're at the stage of typing "two storey extension" into Google at 11pm with a cup of tea going cold beside you, you're probably somewhere between excited and slightly overwhelmed. That was me a few months back, trying to work out whether adding a full second floor to our side return was actually realistic.

Along the way I spoke to a few firms, including Extension Architecture, a RIBA Chartered practice with 17 years of experience and over 1,800 planning approvals across London and Surrey, and pulled together the two storey extension facts that actually mattered once I understood them properly. Here's what I learned.

It's Not Just "Double the Cost" of a Single Storey Job

My first assumption was that a two storey extension would cost roughly twice what a single storey one would. That's not quite right. The roof and foundations get shared across both levels, so the cost per square metre often works out slightly better than you'd expect, even though the total project cost is naturally higher because you're adding more overall space. Realistic budgets for a full double storey extension in London currently sit somewhere between £130,000 and £180,000, depending on size, finish, and how complicated the existing structure is.

The Structural Side Is Where It Gets Serious

This was the bit that actually surprised me most. A single storey extension mostly just needs to support its own weight. A two storey one has to carry the load of an entire upper floor and roof, which means proper structural calculations aren't optional. If you're talking to architects, ask directly whether structural engineering is handled by their own team or outsourced to someone else, it's apparently something only around 1 in 20 firms actually keep in house, and it changes how quickly things get sorted if a calculation needs adjusting partway through.

Planning Permission Almost Always Applies

Unlike some single storey extensions that can sneak through under permitted development, most two storey extensions need a full planning application, especially once you factor in things like the 45 degree rule and how close you're building to a boundary. Councils in London generally aim for an 8 week decision on standard applications, stretching to around 13 weeks if you're in a conservation area or the project is more complex.

Timeline-Wise, Give Yourself More Room Than You'd Think

From first design conversation through to a finished build, a two storey extension typically takes somewhere between 5 and 8 months once construction actually starts, and that's before you count the weeks spent on drawings and getting planning permission sorted beforehand. I'd mentally budget closer to a year from "let's do this" to moving furniture back in.

What I'd Actually Ask an Architect

A few questions I found genuinely useful once I started comparing firms:

  • Is structural engineering handled in house, or will I be coordinating a separate engineer myself?

  • How many two storey projects like this have you actually completed nearby?

  • What's realistic for the 45 degree rule on my specific plot?

  • Will I need a party wall agreement with my neighbours, and what does that actually involve?

Where I've Landed

The biggest lesson has been that a two storey extension is a genuinely bigger undertaking than a single storey one, structurally, financially, and in terms of planning, not just "the same thing but taller." Getting the right people involved early, particularly on the structural side, made the whole process feel a lot less daunting once I understood what was actually involved.

If you're at the same stage I was a few months ago, it's worth having a proper conversation with an architect before you get too attached to a specific design. A free feasibility chat costs nothing and tells you fairly quickly whether your idea is realistic for your specific plot.

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