Do I need an architect for a house extension?

Not legally, but for anything beyond a small permitted development extension, an architect's drawings are usually needed for planning and building regulations. An architect also helps avoid costly design mistakes before building work starts.

How long does a house extension take from design to completion?

Design and planning typically take a few months. Build time depends on size and complexity, a single storey side return is far quicker than a two storey rebuild.

Can I extend without planning permission?

Many single storey rear and side return extensions qualify under permitted development. Larger extensions, wrap-arounds, and properties in conservation areas or listed buildings usually need a full application.

Do I need a party wall agreement for a house extension?

If your extension involves work on or near a shared wall or boundary with a neighbour, you'll usually need a party wall agreement under the Party Wall Act. We flag this early so it doesn't hold up the build once work is ready to start.

Does a house extension add value to my home?

A well-designed extension usually increases a property's value, though the actual return depends on the local market, the quality of the finish, and how well the new space suits the house.

Basement Conversion Architects London

A basement conversion can add a full extra storey to a London home without extending its footprint, but it lives or dies on the structural engineering, waterproofing and party wall process behind it. Design United Studio manages that process from feasibility through to completed build.

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Building Beneath the Surface

The Structural Reality Behind Every Basement Conversion

Digging beneath an existing house means underpinning the foundations, managing groundwater, and keeping a live structure standing throughout construction. It's engineering-led work before it's ever a design exercise, and the sequencing has to be right from the first structural calculation.

We work directly with structural engineers from the earliest stage, so the design and the construction sequence are resolved together as one basement conversion plan, not handed to a contractor to work out later.

Image by Point3D Commercial Imaging Ltd.

Getting the Waterproofing Right the First Time

A basement that leaks isn't a basement; it's an expensive hole in the ground. We specify waterproofing to BS 8102, combining internal and external tanking, cavity drainage and sump pumps to suit the site's actual groundwater conditions rather than a standard default.

Getting this wrong is one of the most common and costly mistakes on basement projects, and one of the hardest to fix once the space is finished and in use.

Managing the Party Wall Process With Your Neighbours

Basement conversions almost always trigger the Party Wall Act, since excavation near a shared foundation affects adjoining owners by law. We coordinate party wall surveyors and notices alongside the design programme, so consent isn't the thing holding up your start date.

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Bringing Natural Light and Ventilation Below Ground

A basement only works as a lived-in space if it doesn't feel like one. Light wells, sunken courtyards and carefully placed openings bring daylight and fresh air into a below-ground room, designed in from the outset rather than retrofitted once the structural shell is fixed.

Navigating London Borough Basement Policies

Several London boroughs, including Kensington and Chelsea and Wandsworth, apply stricter planning policies to basement development than to standard extensions, some limiting depth, footprint or the number of storeys below ground. We check the relevant local policy before finalising a scheme, not after submission.

Why Design United Studio

Ten Years of Taking Below-Ground Extensions Through Planning

Design United Studio is a RIBA and ARB registered practice based in Wimbledon, working across South West London and Surrey.

We take basement and below-ground projects from initial feasibility survey through structural engineering, party wall process and completed build.

Portfolio

Our Track Record

2

Hands-On Team

100+

Projects

99%

Planning Approval Success Rate

10+

Years in Practice

What Our Clients Say

"Excavating under an occupied house sounds terrifying until you have engineers and an architect who've actually done it before. Nothing about our basement conversion felt like guesswork."

"The party wall side alone would have stalled us for months without someone managing it properly. Design United Studio had agreements in place before our contractor was even ready to start."

"We were nervous about losing light in what's technically an underground room. It doesn't feel underground at all."

DAVID R.

EMMA K.

THOMAS B.

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