Go for a walk around Shirley or Selsdon and you'll lose count of the 1930s semis. Bay window, pebbledash or brick, and a long garden running out the back that, let's be honest, is mostly lawn the kids gave up on.

Fill a bit of that garden and you've found your extra room. On these houses a wide rear extension, kitchen knocked through into a proper kitchen-diner, is the job we do more than any other in Croydon. If you're looking at an architect croydon homeowners rate, this is almost certainly the project you're thinking about, even if you haven't put it that way yet.

The houses

Croydon is semi country. Shirley, Selsdon, Sanderstead, Coulsdon, Purley, it's 1930s houses pretty much as far as you can see, and the plots are big. Deeper gardens than you'd get anywhere near central London.

There are the Victorian and Edwardian terraces too, round Addiscombe and South Croydon, and those extend well in their own way. But it's the semi that most people here own, and it's the semi that takes a rear extension best.

The plots are the reason. You can push a decent extension out the back and still have a garden left worth sitting in. That's just not true on a tight inner-London terrace.

What you're actually building

The usual thing is a single-storey extension across the back, opening the pokey old kitchen into one big room. Kitchen down one side, island, table by the garden doors, maybe a sofa in the corner. On a 1930s semi you can normally go back a full 4 metres.

Get a roof lantern over the middle and even a north-facing garden won't leave it dark. That's the part people underestimate, the light, not just the floor space.

On the bigger plots out in Sanderstead and Purley there's room to go further, 5 or 6 metres, which gives you a seriously large room. Worth a look at a house extensions croydon walkthrough to see how these come out on the ground.

The money

Croydon's a bit cheaper than inner London, and that helps. A standard 4-metre rear on a semi comes in somewhere around £45,000 to £75,000, covering the groundworks, the steel, glazing, electrics, plumbing and finishes. A deep one on a larger detached is more like £60,000 to £95,000.

Then there's the kitchen itself. That's a separate budget, and it's the thing that catches people out, because nobody mentions it until the build's nearly done. Factor it in from the start.

Here's why the sums work, though. House prices in CR0 and CR2 are lower than inner London, but the build costs are much the same. So the extension is a smaller chunk of what the house is worth, and the money tends to come back when you sell.

Planning, which is mostly fine

No borough-wide Article 4 here stripping your rights, so that's one worry off the table. A single-storey rear up to 3 metres on a semi, or 6 under the prior approval route, usually counts as permitted development. Lawful Development Certificate and Building Regs, no full application.

One thing that's changed and trips people up. Croydon binned its old householder design guide, the SPD2 rules you'll still find quoted all over the internet. Extensions now get judged against Local Plan policy DM10, and there's no fixed 45-degree line or set depth any more. A good design has more freedom. A lazy one is easier to refuse. So it's worth getting the design right rather than just maxing out the dimensions.

The catch is the conservation areas, 21 of them across the borough. In those, permitted development shrinks and you're into a full application. Check whether your street's in one before you get too far.

The stuff that actually bites

Knocking the back wall out means a steel beam, sized by an engineer. No way round it, and it's a real cost. On a semi you'll need a party wall agreement with next door too, so get that notice served early or it sits there holding everything up.

And a Croydon-specific one worth knowing: a lot of these 1930s houses are on a concrete raft, and often the new extension can tie straight into it. That can make the groundworks simpler and cheaper, so it's worth checking at the start.

So, before you ring an estate agent

Go stand at the back door and look at the garden properly. If most of it is space nobody really uses, your extra room is sitting right there, and it'll cost you less grief than moving.

That's the honest pitch for extending a Croydon semi. Deep gardens, friendly planning on most streets, and a house that was more or less built to grow out the back. Extension Architecture is a RIBA chartered and ARB registered practice working across London and Surrey including Croydon, with in-house structural engineering, so the beam, the raft and the party wall get sorted together instead of surprising you one at a time.