Structural Design for Extensions & Lofts in London
A well-designed structure is the difference between an extension that sails through building control and one that stalls on site. Wimbledon Surveyors provides complete structural design for extensions and loft conversions across London — beams, foundations, connections and the calculations to prove them. Request a design quote.
Rear and Side Extensions
We design the steelwork, lintels and foundations for single and double-storey extensions, coordinating with your architect’s drawings so the structure supports the spaces you actually want.
Loft Conversions
Loft conversions concentrate load into new floor beams and often bear on the party wall. We design the floor structure, dormers and steels — and flag Party Wall Act notices where beams enter shared walls.
Foundations and Ground Conditions
London’s clay soils and street trees affect foundation design. We specify depths and details that satisfy building control and avoid future movement problems.
From Drawings to Site
You receive design drawings and calculations your builder can price and build from, with engineer support available if conditions on site differ from expectations.
Start Your Project Right
Send us your plans and we’ll quote for the structural design your project needs. Contact the team today.
Structural Design for Extensions and Loft Conversions
An extension or loft conversion is only as good as the structure holding it up. We provide complete structural design packages for home extensions, loft conversions and dormers across London and Essex — foundations, steelwork, floor structure, roof alterations and every calculation Building Control needs — working hand-in-hand with your architect and builder from planning through to completion certificate.
Loft Conversion Structural Design
Lofts are structurally demanding: existing ceiling joists were never designed to be floors, and cutting into the roof changes how it works. Our packages cover new floor beams spanning between party walls, steel ridge beams for vaulted spaces, dormer framing, purlin removal and rafter design, staircase trimming, and the party wall implications of beams bearing on shared walls — which we can handle through our party wall notice service in the same instruction.
Single and Double-Storey Extensions
For rear, side-return and wrap-around extensions we design foundations appropriate to London clay (including near trees), steel goalposts and picture-frame openings for glazed elevations, flat and pitched roof structure, and the beams needed where the rear wall of the house is opened up — coordinating with our load-bearing wall removal design where the existing structure is altered.
What You Receive
- Full structural calculations to Eurocodes for Building Control.
- Structural drawings, sections and connection details your builder can price and build from.
- Foundation design including depth requirements near trees and drains.
- Beam schedules with steel sizes, padstones and bearings specified.
- Ongoing responses to Building Control and contractor queries, included in the fee.
Working with Your Architect and Builder
Architects design the space; we make it stand up — and the earlier we are involved, the cheaper the structure usually is. Reviewing a scheme before planning submission lets us flag structurally expensive features (long cantilevers, floating corners, large clear spans) while they can still be adjusted. During construction we answer buildability questions, and can inspect critical stages such as foundations and steel installation before they are covered up.
Adding Value with Confidence
A well-executed loft or extension remains one of the most reliable ways to add space and value in London. Proper structural design protects that investment: Building Control sign-off, no cracking or deflection problems later, and a clean paper trail for your eventual sale. If you are still deciding whether to buy a property with conversion potential, our building surveys can assess feasibility before you commit.
Frequently Asked Questions
Yes. New floor structure, roof alterations, dormers and staircase openings all require engineered design and calculations for Building Control. No compliant loft conversion happens without an engineer’s package.
Design and calculation packages for typical London single-storey extensions usually cost £500-£1,000; loft conversions similar or slightly more; double-storey and complex glazed schemes are quoted individually. Fees are fixed and agreed before we start.
Ideally before planning submission, so structurally expensive features are identified early — and no later than the start of Building Regulations drawings, since our calculations and details form part of that application.
In terraced and semi-detached houses, almost always: floor beams typically bear into the party walls, which is notifiable work under the Party Wall etc. Act 1996. We provide the structural design and serve the notices together, keeping the programme aligned.
Yes — we offer stage inspections at foundations, steel installation and other critical points, giving you independent confirmation the structure is built as designed before it is covered up.