Building Regulation Drawings for a Single-Storey Extension

Flat Roof Construction

Flat roofs are very common on contemporary rear extensions.

A Building Regulations section may need to show:

• structural joists;
• structural support;
• roof deck;
• vapour-control layer;
• insulation;
• waterproof covering;
• internal plasterboard;
• roof falls;
• outlets;
• guttering;
• flashings; and
• abutment details.

The exact build-up depends on whether the roof is designed as a warm roof, cold roof or another suitable system.

A simple planning note saying:

“New flat roof”

does not provide enough technical information to construct the roof correctly.

Pitched Roof Construction

A pitched roof requires different details.

The drawings may identify:

• rafters;
• ceiling joists;
• ridge;
• purlins where applicable;
• restraint straps;
• insulation;
• breathable membrane;
• battens;
• tiles;
• ventilation;
• flashings;
• eaves;
• guttering; and
• internal finishes.

Structural calculations may be necessary where roof members or supporting beams cannot be determined through suitable standard design guidance.

Rooflights

Rooflights can affect several technical issues, including:

• structural trimming;
• thermal performance;
• waterproofing;
• ventilation;
• safety glazing; and
• roof construction.

The opening must be properly coordinated with the roof structure.

Where structural rafters or joists are interrupted, trimming members may need to be designed.

Structural Calculations for a Single-Storey Extension

This is one of the most commercially important parts of the article because many extension projects require structural calculations.

Not every extension needs a large steel beam.

But calculations are commonly required where the work involves:

• removing the existing rear wall;
• widening an existing opening;
• installing bifold doors;
• installing large sliding doors;
• supporting masonry above an opening;
• creating an open-plan kitchen/dining room;
• unusual roof spans;
• structural columns;
• non-standard foundations; or
• other significant structural alterations.

This is where Building Regulations drawings and structural engineering meet.

Do I Need an RSJ for My Extension?

Potentially.

One of the most common extension designs involves removing much of the original rear wall of the house.

The wall may currently support:

• the first-floor wall;
• floor joists;
• roof loads;
• masonry; or
• a combination of loads.

When the wall is removed, those loads still need support.

A steel beam — often casually called an RSJ — is a common solution.

The required beam cannot be selected purely by measuring the opening.

Structural design may need to consider:

• clear span;
• dead loads;
• imposed loads;
• masonry above;
• floor loads;
• roof loads;
• deflection;
• beam reactions;
• supporting walls;
• padstones;
• columns; and
• foundations.

That is why structural calculations are often required.

Do Bifold Doors Need Structural Calculations?

The doors themselves do not necessarily create the need for calculations.

The structural opening above them does.

Large bifold or sliding-door openings can leave very little masonry supporting the structure above.

The design may therefore require:

• steel beam;
• proprietary lintel;
• steel columns;
• goalpost frame;
• box frame; or
• another structural solution,

depending on the circumstances.

The Building Regulations drawings should coordinate this with the structural calculations rather than leaving the contractor to decide the support arrangement on site.

What Are Padstones?

A steel beam transfers concentrated loads into the walls beneath its ends.

A padstone is commonly used to spread these concentrated reactions into the supporting masonry.

The required size and specification depend on the structural design.

A drawing might identify:

PS1 — padstone to engineer's calculations

while the structural package provides the actual design.

Do I Need Steel Columns?

Not always.

Many beams can bear directly onto suitable masonry.

However, columns may be necessary where:

• the available wall is insufficient;
• the reactions are high;
• openings are very wide;
• there is little remaining masonry;
• a frame is required for stability; or
• the overall structural arrangement makes columns necessary.

This is exactly why the structural design should be completed before key supporting walls are removed.

Fire Protection to Structural Steel

Where structural steelwork requires fire resistance, the technical package should show how that protection is intended to be achieved.

Suitable systems can include tested fire-resistant boards, encasement or other appropriate solutions.

Approved Document B provides the principal fire-safety guidance in England. Current published guidance incorporates amendments applying at different dates, including further changes scheduled for 30 September 2026, so the applicable version and transitional provisions matter.

This is another reason the Building Regulations package should reflect the actual project and the applicable standards rather than rely on a legacy template.

Fire Safety for a Single-Storey Extension

Fire safety in a simple ground-floor extension can sometimes be relatively straightforward, but it should not simply be ignored.

Relevant considerations may include:

• means of escape;
• smoke and heat detection;
• new internal layouts;
• fire resistance of structural elements;
• separation from boundaries;
• glazing;
• and the effect the extension has on existing escape arrangements.

For example, extending a kitchen or creating a large open-plan area may change the internal layout of the dwelling.

That can become more significant in houses with upper storeys.

Approved Document B is the principal government guidance for fire safety in dwellings in England.

Ventilation for a Single-Storey Extension

Ventilation is particularly relevant where the extension contains a:

• kitchen;
• utility room;
• WC;
• shower room; or
• bathroom.

The design may need to address:

• mechanical extract ventilation;
• background ventilation;
• purge ventilation;
• duct routes; and
• external terminals.

The government has published a new 2026 edition of Approved Document F for buildings subject to the 2026 standards, while previous guidance continues to apply where earlier regulatory standards remain applicable.

The actual ventilation solution should therefore be coordinated with the building and the standards applying to the work.

Kitchen Extensions and Extract Ventilation

Kitchen extensions are particularly common.

Depending on the arrangement, technical information may need to address:

• cooker extract;
• mechanical extraction;
• background ventilation;
• purge ventilation;
• duct route;
• external discharge; and
• interaction with the wider dwelling ventilation strategy.

This should be considered during design rather than waiting until the kitchen installer arrives.

Windows and External Doors

New windows and doors can involve multiple Building Regulations requirements.

These may include:

• thermal performance;
• safety glazing;
• ventilation;
• structural support;
• means of escape where relevant;
• guarding;
• weather resistance; and
• thresholds.

Large glazed doors are common in rear extensions, so coordinating the structural opening, thermal performance and safety requirements is important.

Drainage for a Kitchen Extension

If the extension contains a kitchen, utility room or WC, the drainage drawings may need to identify new connections.

For example:

Kitchen sink

↓

Waste pipe

↓

Existing or new drainage connection

A toilet or utility room may require additional foul drainage design.

The design should ensure suitable access for inspection and maintenance where required.

Approved Document H provides the core guidance for foul water drainage and disposal in England.

Rainwater Drainage

The new roof also needs somewhere to drain.

Depending on the property, this might involve:

• existing surface-water drainage;
• a soakaway;
• another appropriate drainage arrangement; or
• connection into an existing lawful system.

Drawings may identify:

• gutters;
• downpipes;
• outlets;
• drainage routes; and
• soakaway location where relevant.

Do I Need a Soakaway?

Not every extension requires a new soakaway.

The appropriate method depends on the existing drainage arrangements and site conditions.

Where infiltration drainage is proposed, factors may include:

• soil permeability;
• available garden area;
• proximity to buildings;
• existing drainage; and
• site layout.

This should be considered as part of the project rather than simply assuming that a soakaway will always work.

What About Existing Manholes?

A common problem occurs when a proposed extension covers an existing inspection chamber.

The correct response depends partly on whether the pipe is private or part of the public sewer network and the drainage arrangement.

Possible implications can include:

• relocating the chamber;
• altering drainage;
• foundation coordination; or
• obtaining separate consent where required.

Identifying drainage before building work begins can avoid expensive redesign later.

Electrical Work

Electrical work associated with the extension must also satisfy the applicable requirements.

This can include:

• new socket circuits;
• lighting;
• kitchen electrical work;
• exterior lighting;
• smoke or heat alarms; and
• other electrical installations.

For dwellings, Approved Document P provides guidance on electrical safety.

Some work can be self-certified through an appropriate registered competent person scheme.

Do I Need SAP Calculations for an Extension?

Not every ordinary extension automatically requires a full SAP assessment in the same way as a new dwelling.

However, Part L can require additional energy assessment in particular circumstances.

One example is where the total area of proposed glazing exceeds the normal allowance under the guidance.

Alternative approaches may involve demonstrating that the extension's overall performance is no worse than an appropriate reference arrangement.

The correct assessment depends on the design and the applicable version of Part L.

For homeowners, the practical point is simple:

very large areas of glass can affect Building Regulations compliance even where the glazing is architecturally desirable.

This should be considered before the doors and rooflights are ordered.

Can I Have Large Bifold Doors and Rooflights?

Often, yes.

But extensive glazing can affect:

• heat loss;
• overheating;
• structural support;
• safety glazing;
• roof structure; and
• Building Regulations energy calculations.

A design with almost entirely glazed rear walls plus large rooflights therefore needs more consideration than a conventional extension with modest openings.

Planning permission alone does not establish that the thermal design complies with Building Regulations.

Do I Need a U-Value Calculation?

Potentially.

The drawings need to demonstrate a suitable thermal construction for the proposed elements.

This may involve specifying the materials and build-ups to meet the relevant performance standards.

More complicated arrangements can require calculations or specialist evidence.

What Happens Where the New Extension Meets the Existing House?

Junctions between new and existing construction are among the most important details in an extension.

Examples include:

• new roof against existing external wall;
• new cavity wall against existing masonry;
• new floor against existing floor;
• DPC and DPM junctions;
• insulation continuity;
• steelwork within existing walls; and
• changes in floor level.

Poor detailing at these junctions can contribute to:

• damp;
• cold bridging;
• cracking;
• water ingress; or
• construction difficulty.

A good Building Regulations package should therefore address the extension as an alteration to an existing building, not as an isolated box.

Does a Single-Storey Extension Need a Full Plans Application?

Not necessarily, but Full Plans is often a sensible route for substantial extension work.

For ordinary work that is not within the higher-risk building regime, GOV.UK currently identifies three main application routes:

Full Plans

This is described as the most thorough option. A decision is generally expected within five weeks, or two months with consent.

Building Notice

This is available only for smaller projects. Work can generally start two days after submission, but there is no formal approval of plans.

Regularisation

This is the retrospective route for qualifying work already carried out without the required approval and is available only through a local authority Building Control body.

For an extension with structural steel, insulation details, drainage alterations and other technical issues, preparing the technical package before construction can provide much greater clarity.

Building Control: Local Authority or Registered Building Control Approver?

For ordinary domestic projects in England, Building Control may be undertaken by:

• the local authority Building Control body; or
• a registered building control approver where permitted.

GOV.UK confirms that the application route differs depending on which Building Control body is used, including the use of an initial notice when a registered building control approver is appointed.

The technical design still needs to address the applicable Building Regulations requirements whichever appropriate Building Control route is selected.

Do Building Regulation Drawings Need to Be Approved Before Work Starts?

This depends on the application route.

Under a Full Plans application, the purpose is to submit the technical design for assessment.

Under a Building Notice route, formal approval of plans is not issued beforehand.

However, simply being legally able to commence work does not mean it is sensible to start without the major technical details being resolved.

For example, before demolition begins, the builder should ideally know:

• what supports the house;
• what steel is required;
• what foundations are proposed;
• how the roof is built;
• what insulation is required; and
• what happens to the drains.

Designing these matters before construction generally gives the project a much clearer technical basis.

Can I Use My Planning Drawings for Building Regulations?

Your planning drawings can often form the starting point, but they will usually need considerably more technical information.

A planning drawing might show:

4-metre rear extension with flat roof and bifold doors.

The Building Regulations package then has to resolve:

FoundationHow is it supported?

WallWhat is the actual construction?

FloorHow is it insulated and damp-proofed?

RoofHow are the joists, insulation and waterproofing arranged?

Rear openingWhat supports the existing house?

Bifold openingWhat supports the new wall or roof above?

DrainageWhere do waste and rainwater go?

VentilationHow does the new kitchen ventilate?

That is the difference between planning design and technical design.

I Already Have Planning Permission — What Do I Do Next?

This is exactly where BuildRegs fits into the process.

If your extension has already been designed for planning, you can upload the drawings you have.

BuildRegs will review the information and provide a fixed quotation for the Building Regulations stage.

Where required, the package can include both:

Building Regulation drawings

and

Structural calculations

so you do not have to separately coordinate two unrelated sets of information.