Victorian House Extensions: A Complete London Guide

I've been extending Victorian houses across London for over two decades now, and I can tell you this: no two jobs are ever the same. That's what makes them brilliant to work on – and occasionally frustrating. If you're thinking about extending your Victorian terrace or semi, this guide will give you the honest picture of what to expect.

Understanding Your Victorian House

Before we talk about extensions, you need to understand what you're working with. Victorian houses in London were built between roughly 1837 and 1901, and the construction methods varied depending on the era and the builder's budget.

Most Victorian terraces have solid brick walls – typically 225mm (9 inches) thick for the main walls. There's no cavity, no insulation, just brick. The foundations are usually shallow strip foundations, sometimes only 450mm deep, which can create headaches when you want to dig deeper for an extension.

Typical Victorian House Anatomy

• Solid brick walls: No cavity, typically 225mm thick
• Shallow foundations: Often only 450-600mm deep
• Timber floors: Joists sitting on sleeper walls or directly in pockets in the brickwork
• Rear addition: Usually a two-storey outrigger with the scullery below and bedroom above
• Cellar: Common in earlier Victorians (1850s-1870s), less common later
• Bay windows: Often with their own foundations that need careful consideration

Extension Options for Victorian Houses

Let's look at what actually works on a Victorian property. Each option has its place, depending on your plot, your budget, and what you're trying to achieve.

*Costs are for London in 2026 and include VAT. Actual costs vary significantly by borough, specification, and site access.

The Side Return: London's Favourite Extension

If there's one extension I've built more than any other in the past ten years, it's the side return infill. Most Victorian terraces have this narrow strip of outside space running alongside the kitchen – typically 900mm to 1.5 metres wide. Filling it in can transform a galley kitchen into something genuinely usable.

In boroughs like Hackney, Islington, Lewisham, and Lambeth, I've done hundreds of these. They work because they don't eat into garden space, they usually fall under Permitted Development, and they make an immediate difference to how the kitchen feels.

The secret to a good side return is the rooflight. A long, slim rooflight running the length of the extension brings light into what would otherwise be a dark middle section. I typically recommend at least 2 metres of glazing.

Heritage and Conservation Area Constraints

Here's where Victorian house extensions get complicated. A significant proportion of London's Victorian housing stock sits within conservation areas. In my experience, about 40% of the jobs I quote in inner London have some form of heritage constraint.

What Conservation Areas Mean for Your Extension

• Permitted Development is restricted: You'll likely need planning permission for work that would otherwise be PD
• Materials matter: Councils often insist on matching brick, traditional windows, and slate or clay roof tiles
• Front elevations are protected: Forget about changing windows or doors at the front without approval
• Roof alterations scrutinised: Dormer lofts may be refused or heavily conditioned

Listed buildings are another level entirely. If your Victorian house is listed (Grade I, II*, or II), even internal alterations may need Listed Building Consent. I always tell clients: budget an extra 3-4 months and £5,000-£15,000 in fees for the planning process on a listed property.

Common Structural Challenges

Every Victorian extension throws up challenges. Here are the ones I see most often:

1. Party Wall Agreements

If you're in a terrace or semi-detached, you'll almost certainly need a Party Wall Agreement with your neighbours. This isn't optional – it's required under the Party Wall Act 1996. Budget £700-£1,500 per neighbour if they appoint their own surveyor (which they're entitled to do, at your expense).

My advice: speak to your neighbours early, explain what you're planning, and give them plenty of notice. Most disputes I've seen happen because of poor communication, not the actual building work.

2. Underpinning and Foundation Depth

As I mentioned, Victorian foundations are shallow. When we dig for a new extension, we often need to go deeper than the existing footings. Options include:

• Traditional underpinning: Digging out and pouring concrete in sections beneath the existing wall
• Pad foundations with a ground beam: New foundations that span over the old footings
• Mini piles: For sites with difficult access or problem ground

3. Removing Internal Walls

Most clients want to open up the ground floor – knocking through from the front room to the back, or removing the wall between the kitchen and dining room. In Victorian houses, these walls are often loadbearing.

You'll need a structural engineer to design a suitable steel beam (RSJ). For a typical through-room in a terrace, expect the steel and installation to cost £2,500-£6,000 depending on the span and load.

4. Services and Drainage

Victorian drains are clay pipes, often running diagonally across the garden in ways that made sense in 1890 but are awkward today. A CCTV drain survey (around £200-£350) before you start is money well spent. If drains need diverting, that's £2,000-£5,000 depending on complexity.

Gas and electricity meters are often in awkward positions too. Moving a gas meter costs around £800-£1,500; electric meters are £300-£600. Factor this into your budget if the extension affects their location.

What I'd Avoid

After all these years, there are certain things I steer clients away from:

• Basements in areas with high water tables: Parts of south London (Lewisham, Greenwich) and areas near the Thames can be problematic. The waterproofing costs spiral.
• Extensions that leave no garden: You might get planning permission, but you'll regret it. Families need outside space, and it hurts resale value.
• Cheap flat roofs: A single-ply membrane roof costs more than felt, but it'll last 25+ years instead of 10. Worth the extra £1,500-£3,000.
• Skimping on glazing: Cheap bi-folds and skylights leak. Budget properly for good aluminium frames and quality rooflights.

Timeline Expectations

For a typical rear extension on a Victorian house in London, here's what a realistic timeline looks like:

• Design and planning: 2-4 months (longer for conservation areas)
• Building regulations and party wall: 1-2 months
• Finding a builder and scheduling: 1-3 months (good builders are booked ahead)
• Construction: 10-16 weeks for a single-storey rear extension

From first conversation to moving back into a finished kitchen, you're looking at 9-15 months. Anyone who promises significantly faster is either cutting corners or has no current work – neither is a good sign.