What Most Homeowners Get Wrong About Double Storey Extensions in London

 

Most people start a double storey extension thinking it's basically a single storey job with another floor stacked on top. That's the bit that gets expensive fast. The second floor changes everything about how the build sits on your existing house, how it feeds into the planning conversation, and how much your architect needs to think about before a single line gets drawn.

We see this pattern a lot. A family in Wandsworth or Putney rings up after speaking to a builder, already convinced they know the scope. Then we walk through the structural side, the planning side, the layout shift that happens upstairs, and the room they planned suddenly looks very different.

If you want a feel for how these jobs come together properly on the ground, our double storey extension in Tadworth shows the full process the way we run it for clients, from first sketch through to the finished build.

Why a Double Storey Extension Isn't Just a Bigger Single Storey Job

A single storey extension extends the ground floor and that's roughly that. A double storey one carries weight, ties into existing walls at two levels, and changes the look of your house from the street. Planning officers treat it very differently.

The roof line matters here. So does the position of bedroom windows next door. You can't just push the build up without thinking about overlooking, light loss, and the 45 degree rule that most London councils apply to side and rear elevations. Many homeowners don't realise this until the first planning objection lands on the doormat.

The Planning Permission Bit People Always Underestimate

A lot of clients assume permitted development covers a double storey rear extension. Mostly it doesn't. The PD rules for rear additions are tight on height, depth, and roof pitch, and on most semis or terraces in London you'll need a full householder planning application.

That changes the timeline straight away. Eight weeks for a decision, sometimes more if the council asks for amendments. Conservation areas in places like Richmond, Battersea or Greenwich add another layer, and listed buildings are a whole separate conversation. An experienced architect frontloads all of this so you're not finding out about it three months in.

What a Good Architect Actually Saves You in This Build

People sometimes ask if they can skip the architect and go straight to a design and build firm. You can. But on a double storey job, the architectural decisions feed every other cost in the project, and most of those decisions get locked in during the first month.

A RIBA qualified practice will sketch a scheme that cuts steelwork. It uses the existing chimney breast or party wall instead of fighting it. It sites the new staircase so you don't lose a bedroom upstairs to landing space. These things sound small on paper but they show up as real differences in the finished build.

How the Structural Side Quietly Decides Your Budget

The structural engineer is the person who decides how much steel goes into your house. On a double storey extension that's often a lot more than people expect, because the new first floor needs to span clean and the ground floor below has to carry it.

Having a structural engineer in house, the way we do, means the architect and engineer are sketching together from day one. It's part of the reason our work has been featured on Channel 4 and in the Daily Mail. Beam sizes shrink. Foundation depths sit at sensible numbers. You won't get the awkward situation where the engineer arrives at week ten and says the kitchen island has to move two metres because of a column.

Layout Decisions That Make or Break the Extension

Upstairs is where double storey extensions either succeed or quietly disappoint. A new bedroom that opens off a long awkward landing feels like an afterthought. A proper master suite with a small dressing area and an ensuite changes how the family uses the house every single morning.

Ground floor wise, the question is usually how far back you push and what happens to the kitchen. Open plan kitchen diners with a separate snug at the front of the house is the layout most London families ask for now. Sliding doors at the rear, roof lights over the dining area, a utility tucked off the side. That's the brief in nine out of ten projects we run.

The Timeline Most Families Get Wrong

A lot of clients walk in hoping for a four month turnaround. The reality is different once planning, party wall agreements and building regs are factored in. The build itself is only one slice of the whole programme.

From the first conversation to handover, a double storey extension usually takes 6 to 8 months when it's run well. Planning eats two of those, building regs and tender another month, and the build itself sits around four to five months depending on weather and complexity.