Where is Constructum Code based and where does it carry out construction work?

Constructum Code Ltd has its registered legal and correspondence address in Loughton, Essex. Its operational construction work is delivered on client sites across South East London, Greater London and surrounding areas; the Loughton address is not presented as a public project office or construction site.

House Extensions in London

Technical standards for house extensions in London

A house extension must satisfy more than its architectural brief. Foundation conditions, structural alterations, drainage, thermal performance and neighbouring property rights can all affect the construction specification. We build to the appointed design team’s drawings and coordinate the relevant construction and building control requirements. Our technical approach distinguishes planning permission, Building Regulations approval and party wall procedures, because each serves a different purpose.

Key regulatory checks before construction

Structural design and building control: removing load-bearing walls or extending a dwelling normally requires Building Regulations approval, even where planning permission is unnecessary. We coordinate the structural construction works and applicable inspections against the agreed technical documentation.
Permitted development: side extensions under Class A are subject to conditions including a single-storey limit, a maximum height of 4 metres and a width not exceeding half that of the original house. Other restrictions, including designated land and previous alterations, may affect eligibility.
Thermal performance: Approved Document L governs energy efficiency in residential building work. Extension glazing, insulation specifications and thermal junctions must be assessed against the standards applicable to the project, with additional calculations where required.
Neighbouring structures: the Party Wall etc. Act 1996 can apply to qualifying excavation within 3 metres or 6 metres of neighbouring structures. The relevant foundation depths and statutory conditions must be considered, not distance alone.

For the applicable planning framework, see the UK Government’s permitted development technical guidance for householders.

Which technical conditions can change an extension design?

Small differences in boundary position, glazing area or excavation depth can change the technical route for an otherwise similar house extension. We use these conditions to identify which aspects require further design, engineering or statutory assessment before construction. The following decision matrix sets out practical checks relevant to residential extensions in England as of October 2026.

Technical condition	Regulatory benchmark	Our construction assessment
Side extension dimensions	For eligible Class A permitted development, a side extension must be single-storey, no more than 4 m high and no wider than half the original house.	We compare the proposed dimensions with the existing building and confirm the construction scope against the appointed designer’s planning assessment.
Eaves near a boundary	Where an extension is within 2 m of the property’s boundary, permitted development rules generally limit its eaves height to 3 m.	We review the proposed roof construction, wall heights and boundary relationships against the approved design.
Extensive glazing	Under the currently applicable Approved Document L framework, glazing equivalent to 25% of the extension’s floor area is a key starting benchmark, with allowance for qualifying existing openings covered by the extension.	Where the prescriptive allowance is exceeded, we coordinate the relevant thermal calculations and compliant specification with the design team.
Excavation near neighbours	Section 6 of the Party Wall etc. Act 1996 contains separate excavation tests within 3 m and 6 m of neighbouring structures, depending on excavation depth and foundation geometry.	We coordinate the construction information with the appointed engineer and party wall surveyor where statutory procedures apply.
Conservation area designation	Class A permitted development does not generally authorise side extensions in conservation areas. Article 4 Directions may remove additional permitted development rights.	We work to the relevant approved drawings, material specifications and planning conditions provided by the appointed design team.

The numerical limits above are conditional planning or technical tests, not automatic permission to build. A proposal must also meet the other applicable statutory requirements.

When is an architect-led construction specification ready for tender?

A construction quotation is more reliable when the proposed dimensions, structural requirements, material specification and responsibility for statutory approvals are clearly defined. We work with homeowners and appointed architects to establish a buildable scope from the available information. Where essential design details remain unresolved, we identify them before treating the price and programme as final.

Projects suitable for detailed construction pricing

Structural house extensions supported by existing and proposed architectural drawings.
Projects with structural calculations or an identified structural engineer responsible for outstanding design.
Period property alterations where site access, boundaries and existing construction conditions can be assessed.
Projects requiring coordinated structural works, building services, glazing, roofing and internal finishing.

Information that may still need to be resolved

Unconfirmed structural openings, beam sizes, foundation arrangements or load paths.
Undetermined drainage diversions, inspection chamber positions or underground service routes.
Missing planning decisions or unresolved conditions affecting the proposed construction.
Incomplete glazing, insulation or finishing specifications that materially affect cost.

If your project is still at the concept stage, we can work alongside your appointed architect or help you identify the specialist design input needed before construction pricing is finalised.

Construction details that affect long-term extension performance

Once the planning route and main structure are established, the detailed connections between new and existing construction become critical. We pay particular attention to junctions where movement, moisture, heat loss or drainage problems could arise. These details influence how the completed extension performs beyond its visual appearance.

How do we address insulation and thermal bridging?

Thermal bridging occurs where part of the building envelope permits more heat transfer than the surrounding insulated construction. At an extension, relevant junctions include steel supports, floor edges, window reveals and the connection between new and existing walls or roofs.

We coordinate the specified insulation continuity and construction details with the appointed designer and relevant specialists. U-values and any required energy calculations must reflect the applicable version of Approved Document L.

The Ministry of Housing, Communities and Local Government published the 2026 edition of Approved Document L in March 2026. The associated regulatory changes are scheduled to take effect for most relevant building work from 24 March 2027, subject to transitional provisions. Projects started under earlier applicable standards are not automatically subject to every later requirement.

See the 2026 Approved Document L guidance and the Government’s implementation timetable.

How can existing drainage affect foundations and external levels?

Existing drainage runs and inspection chambers can influence foundation positioning, floor levels and the location of new external walls. We consider these constraints before finalising the relevant construction sequence and coordinate any necessary drainage design or alterations with the appointed specialists.

Requirement H3 of the Building Regulations establishes a hierarchy for rainwater disposal: an adequate soakaway or other infiltration system where reasonably practicable, then a watercourse, and then a sewer where the preceding options are not reasonably practicable. Site conditions determine which solution can be used.

We assess the proposed roof drainage and external water management against the approved design so that rainwater is not simply redirected towards the existing building or neighbouring property.

See the UK Government’s sustainable drainage guidance.

How do we approach structural alterations to period homes?

Victorian and Edwardian houses can contain different foundation arrangements, solid masonry construction, older drainage layouts and structural alterations completed during previous refurbishments. These conditions cannot be established reliably from architectural appearance alone.

For a proposed structural opening, we coordinate the construction method with the appointed structural engineer’s design. Temporary support, permanent steelwork, load-bearing connections and the condition of the surrounding masonry must be addressed as appropriate to the actual structure.

Where existing solid walls are affected by insulation or refurbishment works, moisture behaviour and material compatibility also need consideration. We do not treat one insulation system or foundation solution as suitable for every period property.

What do our completed extension projects demonstrate?

Completed projects provide more useful evidence than generic claims about workmanship. Our published residential portfolio documents extensions with different combinations of structural alterations, glazed openings, building services and internal refurbishment. Two London projects illustrate how construction scope changes when an extension is delivered independently or as part of wider property alterations.

Side-return extension in Peckham, SE15

In 2023, we completed a side-return extension in Peckham, incorporating steel-framed glazed doors, a new open-plan kitchen, a ground-floor bathroom and refurbishment throughout the property. The works also included engineered oak flooring and replacement boiler installation.

The published approximate construction cost was £70,000. This is a historical project figure covering its documented scope, not a current standard price for side-return extensions.

The project demonstrates the coordination required when additional floor space, new glazing, plumbing and whole-property finishing form part of one construction contract. Explore our completed Peckham side-return extension.

Combined extension and loft conversion in Tooting, SW17

In 2024, we delivered a rear kitchen extension and loft conversion at a residential property in Tooting. The completed works included an open-plan kitchen, utility room, aluminium bifold doors, engineered wood flooring and additional accommodation within the roof space.

The published approximate construction cost was £135,000, comprising £75,000 for the extension and £60,000 for the loft conversion. These figures relate to the recorded project scope and should not be used as a quotation for another property.

Combining alterations at ground-floor and roof level requires the structural programme, building services and internal finishing to be considered together. View our completed Tooting extension and loft conversion.

What should you prepare before requesting a detailed extension quotation?

A useful construction brief allows us to assess the actual work rather than price an assumed specification. If your architect has already prepared drawings, the next priority is to identify which technical information is confirmed and which details still require design input. You can use the quotation form already provided on this page.

Share your drawings: provide existing and proposed plans, elevations and sections, including the latest drawing revisions.
Identify structural documentation: supply the available engineering calculations, foundation details and steelwork specifications.
Confirm the planning position: provide the relevant planning decision, prior approval information or other supporting documentation where applicable.
Specify your requirements: identify the intended glazing, finishes, building-services alterations and occupation constraints.

We use the available information to determine the matters requiring site assessment or further clarification before agreeing the construction scope. Contract conditions, variations and warranty coverage should also be understood before works are authorised. Our published terms and conditions explain the contractual framework, including the separate warranty policy and its limitations.

Constructum Code is a London main contractor, building house extensions for private clients and on architect-led projects. We work to your architect’s drawings and deliver the build through to final handover, with one team accountable for the structure, the trades, the programme and the approvals.

Our focus is on build quality and precise execution – the junctions, the alignments and the finishing that determine how an extension looks and performs years after the site has been cleared. Where a client has not yet appointed an architect, we can make introductions.

The four kinds of house extension

The right format depends on the shape of your plot, what your borough allows, and how the house was originally built.

1. Side return extensions

The side return is the narrow strip beside the rear outrigger on a Victorian or Edwardian terrace, typically occupied by a drain and little else. Filling it in adds two to three metres of width to the ground floor and turns a corridor kitchen into a room that can take a dining table.

It is the most common extension in London’s terraced housing because it takes very little from the garden. The work involves removing a load-bearing flank wall, so it requires calculated steelwork and, in nearly every case, a party wall agreement with at least one neighbour. Both are worth establishing early in the project.

2. Rear extensions

A rear extension runs across the back of the house and out into the garden, usually to create an open-plan kitchen and dining space with direct access to it.

Depth is the main constraint. Permitted development covers a certain distance without a full application, with a longer allowance available through the prior approval route. The limits vary depending on whether the house is detached, and a number of boroughs apply further restrictions of their own, so the position needs confirming for your specific address rather than assuming the general rule applies.

3. Wraparound extensions

A wraparound combines the side return and the rear extension into a single L-shaped footprint. It produces the largest gain on the ground floor, and it is what most people are describing when they ask for the kitchen, dining and living space to be one room.

It usually requires full planning permission, because the combined footprint exceeds what permitted development allows. The roof is more complex, and the junction where the two sections meet is a detail that needs to be got right – it is a common source of water ingress where it hasn’t been.

4. Double-storey extensions

A double-storey extension adds living space at ground level and a bedroom or bathroom above it within the same programme. Because the foundations and the roof are shared across two floors rather than serving one, the cost per square metre is usually lower than building the same area as a single storey.

The structural demands are higher: deeper foundations, more steel, and a roof that has to meet the existing one cleanly. Planning is also assessed more closely, since height and massing affect neighbouring properties in a way a single storey does not.

Where a loft conversion is also under consideration, it is generally worth delivering both within one programme.

How we work

Survey first: ground conditions, load-bearing walls, drainage runs, boundary positions and site access are established before anything is priced. This is what determines whether a scheme is buildable within the budget it has been costed against.
Approvals: planning is secured by your architect or planning consultant before the build begins. From there we coordinate the building control route, stage inspections and the notifications the works require, and work with the party wall surveyor where one is appointed, in line with current Building Regulations.
Costs agreed upfront: you receive a detailed, itemised estimate before work begins. Where something changes on site – which on period properties it occasionally does – it is priced, explained and agreed with you before the work proceeds.
One team, one programme: foundations, steelwork, brickwork, roof and weatherproofing, followed by services, insulation, plastering, glazing and joinery. On most residential projects this is delivered by the same site team and project manager from start to finish. 95% of our extensions are completed on or before the agreed schedule.
Regular communication: progress updates throughout the build, and direct contact with the project manager running your site.
Minimal disruption: noise, dust, waste and deliveries are managed around the fact that the house remains occupied and the neighbours remain next door, which matters particularly on terraced properties.
Guaranteed on completion: a 10-year warranty covering the structural work, and a 12-month guarantee on first fix – the pipework, cabling and carpentry installed before plastering.
Fully insured and accredited: we carry £10m employers’ liability, £5m public liability and £1m professional indemnity cover. Constructum Code Ltd is a TrustMark registered firm, a certified VELUX installation partner for roof window systems, and registered with Certass and FairTrades.

See the house extensions we’ve completed here.

What clients say

Fantastic experience with the Constructum team. We had a side return and extension completed in Clapham, and the job was done brilliantly.

— Andrew Slorance, Clapham

I’m very happy with the loft conversion and kitchen extension Constructum Code completed at my house in Bromley.

— James Anderson, Bromley

Read the case studies and reviews on Trustpilot

What an extension costs

House extensions in London with Constructum Code start from £59,000 plus VAT.

That figure reflects a straightforward single-storey rear or side return extension on a standard terrace, with normal ground conditions and workable site access.

The main variables are:

Footprint and depth: the largest single factor, and not a linear one – passing the permitted development limit changes the planning route as well as the volume of work.
Ground conditions: shallow Victorian foundations, drainage in an awkward position, or ground requiring underpinning all add cost below ground level.
Structural requirements: wider openings call for larger steel, which in turn can require a concrete pad rather than a padstone.
Roof and glazing specification: there is a considerable difference between a standard rooflight and a structural glazed corner.
Site access: rear or side access reduces cost compared with a property where all materials pass through the house.
Finish level: kitchens, joinery, sanitaryware and flooring can account for around a third of the total on a well-specified project.

We price after the survey, so the estimate reflects the actual conditions on your site.

Frequently asked questions

Do I need planning permission for an extension?

Often not. Many single-storey rear and side return extensions fall under permitted development, which avoids a full application, although the deeper ones require prior approval from the council. Wraparounds and most double-storey work do need full permission. If the property sits in a conservation area or on a street covered by an Article 4 direction, permitted development rights may have been removed. Your architect or planning consultant will confirm the position for your address before the design is developed.

Will I need a party wall agreement?

If the work builds on or against a shared wall, cuts into it, or involves excavation near a neighbouring structure, then yes. That covers almost every side return extension on a terrace. Notice is served on the affected neighbours; if they consent in writing, the process ends there. If they dissent or do not respond, surveyors are appointed and issue a binding award. A dissent does not prevent the project going ahead, but it adds time and cost, which is why notices are best served early.

Can we stay in the house during the build?

For most single-storey rear and side return extensions, yes. The work area is screened off and the rest of the house kept weathertight and usable. The more disruptive stages are the removal of the load-bearing wall between the existing house and the new structure, and any period where the kitchen or main services are offline. Both are identified in the programme so you can plan around them.

How long does it take?

A single-storey extension is typically ten to sixteen weeks on site, depending on size and structural complexity, with design, structural calculations and approvals running for a couple of months beforehand. Wraparound and double-storey projects take longer. A programme is issued alongside the estimate.

Do you work with our architect?

Yes. We build to drawings produced by your architect and work alongside the practice through to completion, coordinating structural input, specialist trades and the approvals process around their design. Where a client has not yet appointed a practice, we can introduce ones we have worked with.

Explore our portfolio of house extensions across South East and Greater London.