Anyone planning a home extension or loft conversion in London soon meets the term design and build, and it is used loosely. Some companies run design, planning and construction as one in-house team; others present a collection of subcontracted services under a single name. The difference shapes cost, accountability and how much of the project the client has to manage.
As an experienced design and build company with a 97% planning success rate, Sovran has an obvious point of view here, but this guide sets out the trade-offs honestly, without assuming one model suits every project. The difference lies in who carries the risk, who is accountable when something changes mid-project, and how much time you personally spend managing the process.
Bottom line
Design and build means one contract, one team and one point of contact from concept to handover. Appointing a design practice and a builder separately can offer more open competition on price, but the client absorbs the coordination risk between the two. For a home extension or loft conversion, the right choice depends on how much oversight you want to hold personally.
What Design and Build Actually Means (and What It Does Not)
Design and build means a single company, such as Sovran, takes responsibility for architectural design, planning strategy and construction under one fixed-price contract. It does not mean a reduced level of design ambition; a design and build practice with in-house architecture still produces full architectural drawings, structural coordination and planning submissions. What it removes is the handover point between a design phase and a build phase, which is where most cost and timeline disputes on a London home extension or loft conversion originate.
What Changes When Design and Construction Are Separate Firms
When a design practice and a builder are appointed separately, the client becomes the connective tissue between two firms who did not choose to work together and are not contractually bound to the same outcome. If a structural detail proves impractical on site, or a specification needs revising once digging begins, responsibility can become genuinely unclear.
Cost Comparison for a London Home Extension: Is Design and Build Actually Cheaper?
On paper, appointing a design practice and a builder separately can appear cheaper, since each fee is negotiated and tendered independently. In practice, the total cost of a London home extension under this model frequently rises through variations, contingency claims and extended professional fees once the two parties are coordinating mid-build. A fixed-price design and build contract, by contrast, prices the full home extension programme, including professional fees, at the outset.
Who Is Each Model Right For? Home Extension and Loft Conversion Projects Compared
Appointing a design practice and a builder separately can suit a client who wants to hold direct creative control over every design decision and has the time to manage two contracts personally, often the case on a highly bespoke, one-off architectural commission. A design and build model tends to suit home extension and loft conversion projects where the client values a fixed price, a single point of contact and a defined programme from design through to handover.
What to Look For in a Design and Build Company in London
Not every company using the term design and build offers genuine in-house architectural capability; some simply subcontract design work to a third party while presenting it as one service. Look for in-house architecture and engineering capability employed directly within the company, a fixed-price contract structure, and a track record of planning approvals specific to London boroughs, rather than a generic national portfolio.
Why In-House Delivery Matters When Choosing a Design and Build Company
In-house delivery means the team responsible for the design is also accountable for its delivery on site. Planning strategy, structural coordination and construction sit with one company under one contract, so questions are resolved inside the team, not passed between firms. Sovran's design and build team, led by former planning officers with a 97% planning success rate, is built around this single point of accountability.
| Factor | Design and Build (Sovran) | Design Practice and Builder, Appointed Separately |
|---|---|---|
| Number of contracts | One | Two or more |
| Single point of contact | Yes | No — client coordinates |
| Cost certainty | Fixed-price contract | Variable, subject to tender and variations |
| Accountability | One team, fully accountable | Responsibility can be contested |
| Typical suitability | Home extensions, loft conversions, full renovations | Highly bespoke, one-off architectural projects |
| Planning strategy integration | Built in from design stage | Depends on the design team, not guaranteed |
Weighing up design and build against appointing separately?
Book a free consultation with Sovran's experienced team. We will give you an honest view of which model suits your home extension or loft conversion, not just a sales pitch for our own service.
Sovran is an experienced design and build company delivering home extensions and loft conversions across London since 2011. Our architectural team includes former planning officers, enabling a 97% planning success rate across all London borough applications.