Single-Storey Extension Building Regulations: Complete Guide

Do Large Bifold or Sliding Doors Cause Building Regulations Problems?

Not inherently.

The issue is whether the complete design satisfies:

• structural requirements;
• thermal requirements;
• glazing safety;
• ventilation;
• drainage;
• thresholds;
• and relevant fire considerations.

A six-metre sliding-door opening, for example, may require:

• substantial structural support;
• carefully designed lintel or steel;
• thermal detailing;
• appropriate glazing;
• suitable threshold construction.

The opening is both an architectural feature and a technical design problem.

Roof Construction

Single-storey extensions commonly use:

• flat roofs;
• pitched roofs;
• lean-to roofs;
• tiled roofs;
• warm roofs;
• cold roofs.

The roof design needs to address:

• structural loading;
• timber member sizes;
• insulation;
• condensation;
• ventilation where required;
• waterproofing;
• drainage;
• fire considerations;
• rooflights;
• thermal bridging.

A roof should therefore be designed as a complete build-up rather than simply showing its external shape.

Flat Roof Building Regulations

Flat roofs are especially common on modern rear extensions.

A typical warm-roof arrangement may include:

• ceiling finish;
• roof structure;
• deck;
• vapour-control layer where required;
• rigid insulation;
• waterproofing system.

The exact sequence depends on the specified roof system.

Warm roofs are commonly used because the insulation is positioned above the structural deck, helping keep the deck warmer and reducing condensation risk when correctly detailed.

Other compliant roof designs are possible.

Do Flat Roofs Need a Fall?

A nominally “flat” roof should still be designed to drain water.

The roof build-up needs an appropriate fall toward:

• gutter;
• outlet;
• or other drainage point.

The design should account for practical construction tolerances so that completed falls provide effective drainage and reduce the risk of ponding.

Rooflights and Roof Lanterns

Rooflights can significantly improve daylight within an extension.

They can also affect:

• roof structure;
• insulation;
• thermal performance;
• structural trimming;
• roof drainage;
• condensation detailing;
• overall glazing area.

Large rooflights may require additional structural members around the opening.

These should be coordinated with the roof-joist design rather than cut into the roof structure later without checking.

Structural Calculations for an Extension

Many single-storey extensions require structural calculations.

Typical structural elements include:

• beams;
• lintels;
• roof joists;
• rafters;
• ridge beams;
• rooflight trimmers;
• steel columns;
• goalpost frames;
• foundations.

One of the most common structural calculations occurs where the existing rear wall of the house is removed.

Removing the Existing Rear Wall

A homeowner often builds an extension and then wants the original rear wall almost completely removed so the extension and existing house form one large room.

That original wall may be supporting:

• first-floor joists;
• masonry above;
• another storey;
• roof loads;
• or a combination of loads.

It may therefore need to be replaced with:

• structural steel beam;
• beam and columns;
• goalpost frame;
• box frame;
• or another engineered solution.

The steel should be designed specifically for the loads and support conditions.

The opening width alone is not enough to determine the beam.

Do I Need an RSJ for My Extension?

Possibly.

If an existing load-bearing wall is removed, a structural beam is often required.

You may also need structural support for:

• wide bifold doors;
• sliding doors;
• roof openings;
• roof lanterns;
• unusual roof spans.

Although homeowners often call every steel beam an RSJ, the engineer may actually specify a Universal Beam, Universal Column or another structural section.

What matters is that the structural member is appropriately calculated.

Lintels Over Doors and Windows

Openings in masonry need appropriate structural support.

Depending on the project, this could be:

• proprietary lintel;
• steel lintel;
• concrete lintel;
• structural steel beam;
• another designed solution.

The lintel needs to suit:

• opening width;
• wall construction;
• cavity;
• loading;
• bearing;
• exposure.

Large sliding-door openings may require substantially more structural design than an ordinary window opening.

Drainage

Drainage is one of the most frequently underestimated extension issues.

The proposed extension may affect:

• foul drains;
• rainwater pipes;
• inspection chambers;
• underground drainage runs;
• public sewers;
• private drainage;
• existing manholes.

Approved Document H provides guidance on foul water, rainwater and drainage systems.

The existing drainage layout should therefore be considered early.

What If There Is a Drain Under the Extension?

This is common.

Possible solutions can include:

• diverting the drain;
• designing the extension around it;
• protecting the pipe;
• altering inspection arrangements;
• or other work agreed with the relevant parties.

The correct solution depends on:

• whether the drain is private or public;
• pipe depth;
• pipe condition;
• direction;
• proximity to foundations;
• sewer owner requirements.

Do not assume that every drain visible in the garden belongs solely to the property owner.

Building Over or Near a Public Sewer

If the proposed extension is over or close to a public sewer, separate sewer-owner requirements may apply in addition to Building Regulations.

This can affect:

• foundation position;
• sewer access;
• manholes;
• structural design;
• drainage layout.

It is therefore sensible to identify drainage early rather than discovering a public sewer after foundation trenches have been excavated.

Can a Manhole Be Left Inside the Extension?

This depends on the drainage arrangement and ownership.

Internal inspection chambers are generally undesirable and can create access, odour and maintenance problems.

Where drainage conflicts with the extension footprint, the design may need to consider:

• relocating the chamber;
• rerouting drainage;
• altering connections;
• or another appropriate solution.

This should be resolved before the ground-floor construction is finalised.

Rainwater Drainage

The new roof creates additional rainwater runoff.

The design therefore needs to consider:

• gutters;
• downpipes;
• outlets;
• soakaways;
• drainage connections;
• roof falls.

Rainwater should be discharged appropriately rather than simply directed toward:

• the neighbouring property;
• the building foundation;
• or an unsuitable part of the garden.

Ventilation

Adding an extension can change the ventilation characteristics of the existing home as well as create new rooms requiring ventilation.

Part F addresses ventilation requirements.

Relevant considerations can include:

• background ventilation;
• purge ventilation;
• mechanical extract;
• kitchens;
• utility rooms;
• bathrooms;
• habitable rooms.

The correct strategy depends on the room use and existing dwelling.

Kitchen Extension Ventilation

If the extension contains a kitchen, suitable extract ventilation will usually need to be considered.

This can involve a mechanical extract system ducted appropriately to outside.

The design needs to account for:

• extraction rate;
• duct route;
• external termination;
• replacement/background air;
• noise;
• cooker position.

A recirculating cooker hood should not automatically be assumed to satisfy the same function as extract ventilation to outside.

Will New Windows Need Trickle Vents?

Background ventilation requirements need to be considered under Part F.

If windows are being installed, replaced or affected as part of the work, the designer should consider how adequate background ventilation will be maintained.

The government guidance also makes clear that simply locking a window slightly open using a night-latch is not treated as an appropriate substitute for background ventilation.

The complete ventilation strategy should therefore be coordinated with the selected window specification.

Does Part O Overheating Apply to an Extension?

For an ordinary extension added to an existing residential building, Part O does not apply.

Part O is directed at reducing overheating risk in new residential buildings.

Government guidance specifically confirms that it does not apply to extensions or conservatories added to residential buildings after those buildings have been constructed.

That does not mean overheating should be ignored as a design issue.

A heavily glazed south- or west-facing extension can still become uncomfortable.

Shading, glazing specification and ventilation may therefore be sensible design considerations even where Part O itself does not apply.

Fire Safety

Fire safety requirements can still be relevant to a single-storey extension.

The implications depend on the existing house and proposed layout.

Issues may include:

• escape routes;
• fire-resisting construction;
• structural steel protection;
• doors;
• windows;
• smoke alarms;
• internal layout.

A straightforward kitchen extension to a conventional two-storey house may have relatively modest fire implications.

A more extensive open-plan alteration could require greater consideration.

Does a Steel Beam Need Fire Protection?

Often, structural steel forming part of the building may require suitable fire protection depending on the circumstances and required fire resistance.

This could involve an appropriate:

• board encasement;
• proprietary fire-protection system;
• or another suitable construction.

The structural calculation determines the beam.

The Building Regulation specification should address the wider fire-protection requirement.

Open-Plan Extensions and Fire Safety

Opening up the ground floor can sometimes affect the escape arrangement from upper floors.

For example, if a traditional hallway and staircase arrangement is altered so that the stair discharges directly into a large open-plan kitchen/living area, fire-safety considerations can become more significant.

This is particularly relevant where the house has:

• several storeys;
• loft accommodation;
• unusual escape arrangements;
• substantial open-plan changes.

The complete house layout should therefore be considered, not just the new extension room.

Windows and Glazing

New windows and glazed doors can be affected by requirements relating to:

• thermal performance;
• safety glazing;
• ventilation;
• fire safety;
• structural support;
• weather protection.

Safety glazing can be required in critical locations where there is increased risk of human impact.

Large areas of glass should therefore be specified as part of the technical design rather than treated purely as an aesthetic choice.

Electrical Work

Relevant electrical work must comply with Part P.

A typical extension might include:

• sockets;
• lighting;
• kitchen circuits;
• appliances;
• external lighting;
• heating controls.

Electrical work may be self-certified by an appropriately registered competent person where the applicable scheme covers the work.

The electrical installation should also be coordinated with the architectural and Building Regulations design.

Heating

An extension needs an appropriate heating strategy.

This might include:

• radiators;
• underfloor heating;
• heat pump system;
• extension of an existing heating system.

The heating design should be considered alongside:

• insulation;
• glazing;
• room size;
• heat loss;
• controls.

Adding a heavily glazed extension without considering its thermal performance can result in a room that is difficult to heat comfortably.

Accessibility and Level Thresholds

The relationship between the existing dwelling and new extension should be carefully considered.

Issues can include:

• floor levels;
• internal steps;
• external thresholds;
• access routes;
• door widths.

Where practical, good design should avoid unnecessarily creating new barriers to movement.

The specific Building Regulations implications depend on the existing dwelling and nature of the work.

Do I Need Building Regulation Drawings?

For most extensions, a proper technical drawing package is extremely useful and is generally the most robust way to coordinate the project.

A Building Regulation drawing package might contain:

Proposed plans

Showing the technical layout.

Sections

Showing:

• foundations;
• floors;
• walls;
• roof;
• insulation;
• levels.

Construction details

Showing critical junctions.

Building Regulation notes

Covering relevant technical specifications.

Structural drawings

Identifying:

• beams;
• columns;
• lintels;
• roof structure.

Structural calculations

Where required.

Drainage information

Showing new and existing drainage where relevant.

The exact package depends on the extension.

Full Plans vs Building Notice

For ordinary residential extension work in England, two routes commonly encountered are:

Full Plans

This is the more thorough route.

Technical drawings and relevant supporting information are submitted to Building Control for assessment.

A formal decision is normally expected within five weeks, or up to two months with consent.

For an extension involving multiple technical elements, Full Plans can give greater certainty because the design is reviewed before significant construction progresses.

Building Notice

A Building Notice can be used for certain smaller projects.

Work can generally start after the required notice period.

However, there is no formal approval of plans in advance.

Building Control can request technical information during the project.

For a significant extension, proceeding without detailed technical drawings can shift more design decisions onto the building stage.

Local Authority Building Control or Private Building Control?

For an ordinary house extension outside the higher-risk building regime, Building Control can generally be provided by:

• the local authority; or
• a registered building control approver.

Either route is intended to assess Building Regulations compliance.

The designer and contractor remain responsible for producing and constructing compliant work.

Building Control does not become the project's designer simply because it checks the design.

What Does Building Control Inspect?

Inspection stages vary by project and Building Control body.

Typical extension inspections can include:

Foundation excavation

Before concrete.

Foundation concrete

Where required.

Oversite / ground floor

Before the floor construction is concealed.

Damp-proofing

Including DPC/DPM arrangements.

Drainage

Before underground pipework is covered.

Structural work

Including beams, bearings and supporting elements where appropriate.

Roof structure

Before concealment.

Insulation

To check relevant construction before finishes are installed.

Final inspection

When the project is complete.

You should agree the inspection requirements with your Building Control body rather than assuming when inspections will occur.

Can Building Control Tell My Builder How to Construct the Extension?

Building Control assesses compliance.

It should not be relied upon as the project's design service.

The project still needs competent:

• designers;
• structural designers where necessary;
• contractors.

A Building Control inspector identifying a problem on site is not a substitute for designing the construction properly beforehand.

The Building Regulations Dutyholder Regime

Building Regulations in England now include formal dutyholder requirements.

The roles can include:

• client;
• designer;
• principal designer;
• contractor;
• principal contractor.

For a domestic extension, the homeowner is normally the domestic client.

People undertaking design and construction work need appropriate competence for the work they undertake.

Where more than one designer or contractor is involved, the Building Regulations also contain arrangements concerning who is in control of the design and construction phases.

These Building Regulations roles should not be confused with similarly named roles under the separate Construction (Design and Management) Regulations.

Is BuildRegs Automatically the Building Regulations Principal Designer?

No.

Commissioning Building Regulation drawings or structural calculations from a designer does not by itself mean that designer has accepted appointment as the Building Regulations Principal Designer.

The project's dutyholder appointments and responsibilities need to be established separately.

The scope of any professional appointment should make this clear.

Do I Need Planning Permission Before Building Regulation Drawings?

Not necessarily.

The two processes are separate.

However, for many extensions it is commercially sensible to establish the planning position before committing heavily to detailed technical design.

A common sequence is:

Planning / permitted development position established

↓

Building Regulation drawings

↓

Structural calculations

↓

Building Control submission

↓

Builder pricing / construction

There are projects where the technical design starts earlier, but the planning and technical design need to stay coordinated.

When Should Structural Calculations Be Prepared?

Ideally before construction reaches the structural work.

If the extension includes:

• rear-wall removal;
• large door opening;
• steel beams;
• roof beams;
• structural columns;
• unusual roof structure;

the calculations should be coordinated with the Building Regulation drawings.

That prevents situations where the drawings show one arrangement while the engineer calculates another.

Common Building Regulations Mistakes With Extensions

Several problems appear repeatedly.

Starting With Planning Drawings Only

Planning drawings may not contain enough construction information.

Guessing Foundation Depths

The site conditions may require something different.

Ordering Bifold Doors Before Structural Design

The required steel and actual structural opening may affect the order.

Ignoring Existing Drains

A drain or public sewer can materially affect the foundation and floor design.

Over-Glazing Without Checking Part L

Large sliding doors and rooflights can create a thermal-compliance issue.

Changing the Structural Layout on Site

Moving a beam, column or opening can invalidate calculations.

Concealing Work Before Inspection

Building Control may need to inspect critical construction stages.

Mixing Uncoordinated Drawings

Architectural drawings, structural calculations and builder changes need to describe the same project.

Can I Start Building Before Full Plans Are Approved?

Depending on the Building Control route and circumstances, it can be legally possible for work to start before a Full Plans decision has been issued.

But that does not remove the risk.

If construction proceeds and Building Control subsequently identifies non-compliant details, completed work may need to be changed.

For that reason, many homeowners prefer to have the principal technical design resolved before substantial construction begins.

What If I Build the Extension Without Building Regulations Approval?

This can create several problems.

The local authority may have enforcement options where Building Regulations have not been complied with.

You may also encounter difficulties when:

• selling the property;
• remortgaging;
• demonstrating the structural adequacy of the work;
• obtaining completion documentation.

For qualifying historic unauthorised work, a local-authority regularisation process may sometimes be available, but it can require:

• investigations;
• opening-up;
• calculations;
• alterations;
• remedial work.

It is generally much easier to deal with Building Regulations properly before and during construction.

What Is a Completion Certificate?

Once the building work has been completed and the Building Control body is satisfied with the relevant requirements and processes, completion documentation can be issued.

This can become important when:

• selling the property;
• refinancing;
• demonstrating that the Building Control process was completed.

Keep the certificate with the property's other important documentation.

How Much Does Building Control Cost?

Building Control fees are separate from the cost of preparing Building Regulation drawings and structural calculations.

Local-authority fees vary according to factors such as:

• type of work;
• floor area;
• required inspections;
• project characteristics.

Registered building control approvers set their own fees.

You should therefore distinguish between:

Design fee

For producing the technical drawings and calculations.

and

Building Control fee

For the regulatory Building Control service.

They are not the same charge.

How Much Do Building Regulation Drawings for an Extension Cost?

Prices vary according to the project and service scope.

Factors include:

• extension size;
• structural complexity;
• number of steel beams;
• roof design;
• drainage;
• quality of existing drawings;
• amount of technical detailing required.

BuildRegs currently offers Building Regulation drawings and structural calculations for suitable single-storey extension projects from:

£799

The project is reviewed before you proceed so the actual scope and fixed price can be confirmed.