Design and Build in London
One company for the design, the approvals, the fixed price and the build of your extension, loft conversion or renovation.
One Team From First Drawings to Handover
At a glance: Alstruct designs and builds house extensions, loft conversions and renovations across London under one contract. Our design team carries out the measured survey and prepares the layouts, planning drawings and 3D visualisations, or we work with your own architect. We then serve the party wall notices, obtain building regulations approval, price the work as a fixed, itemised quote on a JCT contract and build it, with one point of contact from the first visit to handover.
Design and build means one company is responsible for both the design and the construction. Our guide to choosing a builder in London sets out the trade-off: you get a single point of responsibility, the design and the construction preparation can overlap, and the builder's knowledge of construction costs informs the design from the start. You rely on one firm for both, so look at the work it has already delivered.
We start with a site visit to look at the structure, the planning position for your address, your brief and your budget. If you are at an early stage, we are happy to advise on feasibility and give a preliminary budget before you commit to design fees; our house extension cost guide lists the professional fees to allow for. Our design team then carries out the measured survey and develops the layout with you, from first options to the planning drawings, typically over 4–8 weeks for a single-storey extension and 6–10 weeks for a double-storey.
Planning drawings are not the end of the design. The building regulations drawings run alongside on the Full Plans route, so the technical drawings are approved before construction starts, and a structural engineer designs the foundations and steelwork. Party wall notices are served at the same time. With the drawings, the engineer's input and a complete specification in hand, we prepare a detailed scope of works and programme and issue a fully itemised quotation broken down by trade, on the JCT Minor Works Building Contract.
If you already have an architect, we work directly with them, and for planning drawings we can also bring in independent architects we work with regularly. The company is led by an engineer, and a dedicated site manager coordinates every trade and the building control inspections through to the completion certificate. Every project carries our 10-year workmanship warranty from practical completion. Already have drawings? See how we price from a drawing pack.
What We Do
Feasibility and budget
A site visit to look at the structure and the planning position for your address, with advice on feasibility and a preliminary budget before you commit to design fees.
Survey and layouts
A measured survey of the house, then layout options developed and discussed with you before the scheme is fixed.
Planning drawings and 3D visualisations
Drawings for a planning application or a lawful development certificate, with 3D visualisations, and we manage the application. See how long planning permission takes in London.
Building regulations and structure
Technical drawings approved on the Full Plans route before work starts, with structural calculations for the foundations, steelwork and any underpinning.
Specification and scope of works
Material selections agreed with you and a detailed scope of works and programme, so the price rests on a complete specification, not provisional sums for items that have not been properly scoped.
Party wall notices
Notices prepared and served alongside the planning stage so they do not hold up the start. See party wall agreement costs.
Working with your architect
If you already have an architect, or a listed building needs a conservation architect, we work from their drawings and coordinate the structural and building regulations package.
The build
A fixed, itemised price on a JCT contract, then the work itself: house extensions, loft conversions, renovations and joinery, run by one site manager.
Examples From Our Projects
- Stratford Twin Terraces, E15: two adjacent Victorian terraces, with the design developed by our design team and refined with the client before works began. Both houses were refurbished in parallel and handed over together within the 26-week programme. See the project.
- Cockfosters Victorian Semi, EN4: ground and first floor extensions built with the project architect, with a rooflight arrangement over the new kitchen, structural work designed and certified by our structural engineer and Building Regulations approval secured before work started. Completed in about 24 weeks. See the project.
- Golders Green 1930s Semi, NW11: a rear extension and a two-storey side extension under a JCT Minor Works contract, with a full professional team including architect, structural engineer and CDM health and safety coordinator. See the project.
- Limehouse Cottage, E14: the architect issued the joinery and fittings drawings in stages and our site team built to them, a bespoke joinery package worth over £32,000 alongside a two-storey extension and underpinning. See the project.
Planning a project?
Start with a site visit and your brief
Tell us about the house and what you want from it, and attach any drawings or photos you have. The contact form takes PDF, JPG, PNG, HEIC and DWG files.
Send an enquiry Call 020 8080 8240CHAS Elite · Constructionline Gold · FMB member · £10M public liability
How we price
- A fixed price for the agreed scope. After a site visit we send an itemised quote, set out trade by trade and based on a complete specification, with no provisional sums for items we have not properly scoped.
- A JCT contract. The work runs under a JCT building contract, the Minor Works form on our published projects, with a defined programme.
- No change without a written price. If you change the scope, or the work turns out to need more than was agreed, we send a written variation with its cost first, and that work goes ahead only once you have agreed it.
- A 10-year workmanship warranty. It covers our workmanship and starts on the day the finished work is handed over to you.
The full wording is in our terms. For typical London figures, see our cost guides.
Frequently Asked Questions
What does design and build mean?
One company takes responsibility for both the design and the construction under one contract. With Alstruct, our design team prepares the drawings and we manage planning, building regulations and party wall notices, then build the scheme at a fixed, itemised price on a JCT contract. You deal with one company throughout, with one point of contact from the first visit to handover.
Who prepares the drawings?
Our design team carries out the measured survey and prepares the layouts, planning drawings and 3D visualisations, and a structural engineer designs the foundations and steelwork. For planning drawings we also work with your own architect or with independent architects we work with regularly. Either way, the design and structural package is complete before work starts on site.
Can I use my own architect?
Yes. If you already have an architect, we work directly with them and price from their drawings, and we coordinate the structural engineer and building regulations so the package is complete before the build starts. Where a listed building needs a conservation architect, we work with one. If your drawings are ready, you can send the whole pack with your enquiry.
How long does the design stage take?
Typically 4–8 weeks for a single-storey extension and 6–10 weeks for a double-storey, from the measured survey to the planning drawings. Where planning permission is needed, allow a further 8–12 weeks. Building regulations drawings run alongside on the Full Plans route and party wall notices are served at the same time, so they do not hold up the start.
How is a design and build project priced?
Once the drawings, the structural engineer's input and the specification are in place, we issue a fully itemised quotation broken down by trade, on the JCT Minor Works Building Contract. It is a fixed price for the agreed scope, and any change is confirmed in a written variation, with its cost, before the work is done. Our cost guides list the professional fees to budget for.