Building Regulations for Double-Storey Extensions: Complete Guide

Part B — Fire Safety

Fire safety must be considered as part of a double-storey extension.

The extension can alter:

room arrangement;

escape routes;

doors;

windows;

stairs;

protected circulation.

Does a Double-Storey Extension Need a Second Staircase?

For an ordinary domestic house extension:

normally no.

Do not confuse normal house-extension rules with the forthcoming Approved Document B provisions concerning second staircases in residential buildings over:

18 metres.

Those amendments take effect on 30 September 2026, subject to transitional provisions, and are aimed at much taller residential buildings.

They do not mean a typical two-storey house extension suddenly needs two staircases.

Fire Escape

The proposed layout should maintain an appropriate escape strategy.

Issues can include:

route from first-floor bedrooms;

access to final exit;

protected stair arrangements where relevant;

open-plan layouts;

fire-resisting construction.

Extending Over or Around the Existing Escape Route

If a project alters the relationship between:

bedrooms;

stairs;

ground-floor rooms,

the fire strategy should be reviewed rather than assuming the existing arrangement remains acceptable.

Fire Doors

Whether fire-resisting doors are needed depends on the:

dwelling configuration;

layout;

escape strategy.

Do not treat “all extensions require fire doors everywhere” as a universal rule.

Smoke Alarms

Building work can trigger requirements to provide suitable smoke-alarm arrangements.

The alarm strategy should be shown on the technical drawings where relevant.

Steel Fire Protection

Structural steel may need suitable fire protection.

Possible solutions can include:

fire-resisting board systems;

other tested/proprietary protection systems.

The required specification depends on the project and fire-resistance requirement.

Open-Plan Ground Floors

A double-storey extension often creates a large:

kitchen / dining / family room.

If that layout changes the escape route from upper floors, fire-safety implications need to be considered during design.

Do not leave that decision until the extension is almost finished.

Part C — Moisture and Ground Conditions

Approved Document C covers:

site preparation;

contaminants;

sub-soil drainage;

moisture resistance.

For a double-storey extension this can affect:

foundations;

ground floors;

external walls;

roof construction.

Damp-Proof Membrane

A ground-bearing floor commonly requires appropriate protection against ground moisture.

The DPM should coordinate with:

DPC;

wall construction;

floor insulation.

External Ground Levels

Poorly coordinated external levels can cause moisture problems.

The technical details should consider the relationship between:

finished floor;

DPC;

paving;

garden level.

Part L — Insulation and Energy Efficiency

The extension needs to satisfy the applicable energy-efficiency requirements.

This affects:

external walls;

roof;

ground floor;

windows;

doors;

thermal bridging;

heating changes.

Important 2026 Part L Position

As of August 2026, the government has published:

Approved Document L 2026.

However, for ordinary non-higher-risk work, the associated new regulatory standards come into force on:

24 March 2027.

Where the relevant Building Control notice/application is made before 24 March 2027 and the work is commenced before:

24 March 2028,

the earlier standards can continue to apply under the transitional arrangements.

The applicable standard should therefore be checked against the project's:

submission date;

commencement date.

Why This Matters for Extension Projects Starting in 2026–2028

Two otherwise identical extensions could potentially be subject to different Part L/F standards depending on:

when the Building Control process started;

when the work commenced.

Do not automatically apply a future Approved Document to a project simply because it has already been published.

Wall Insulation

External walls need to meet the applicable thermal-performance standards.

The technical drawing should normally identify:

wall build-up;

insulation type;

insulation thickness;

cavity arrangement.

Roof Insulation

The insulation strategy depends on the roof design.

For example:

Pitched Roof With Insulation at Ceiling Level

differs from:

Insulation Following the Roof Slope.

A vaulted roof usually requires more careful thermal detailing.

Floor Insulation

The ground-floor build-up may include:

slab/screed;

insulation;

DPM;

sub-base.

The precise sequence depends on the system.

Thermal Bridging

Areas such as:

wall/floor junctions;

lintels;

openings;

roof/wall junctions

need appropriate detailing to limit unwanted heat loss and condensation risk.

Windows and Doors

New windows and doors need to meet applicable requirements relating to:

thermal performance;

safety glazing;

ventilation;

fire escape where relevant.

How Much Glazing Can a Double-Storey Extension Have?

Under the currently applicable pre-2027 Part L approach for most ordinary existing dwellings, extension glazing is commonly assessed against the familiar:

25% rule.

Broadly, the area of windows, rooflights and doors is considered against:

the new extension floor area;

plus the area of openings in the existing dwelling that are no longer exposed because of the extension.

Where a heavily glazed design exceeds the standard approach, alternative energy calculations may be used to demonstrate compliance.

Does Excessive Glazing Mean the Extension Cannot Be Built?

No.

It may instead require a compensating or whole-house calculation approach.

For example:

better insulation;

improved glazing;

other energy measures

may help demonstrate compliance.

Part F — Ventilation

New habitable rooms and wet rooms require appropriate ventilation.

This can include:

background ventilation;

purge ventilation;

mechanical extract.

The exact requirements depend on:

room type;

ventilation system;

applicable regulatory standard.

Important 2026 Part F Position

Approved Document F 2026 has also been published.

The associated Future Homes and Buildings Standards changes for ordinary non-higher-risk work come into force on:

24 March 2027,

subject to the same broad transition where an applicable notice/application is made before that date and work begins before 24 March 2028.

Kitchens

A new or extended kitchen generally needs suitable extract ventilation.

The technical design should coordinate:

fan/extract location;

duct route;

external termination.

Bathrooms

A new first-floor bathroom or ensuite can affect:

ventilation;

drainage;

hot-water provision;

floor structure.

Bathrooms should therefore be shown clearly on the technical drawings.

Part O — Overheating

A useful point:

Part O does not normally apply to an extension added to an existing residential building.

Government guidance confirms Part O applies to new residential buildings and does not apply to extensions or conservatories added after the dwelling has been built.

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

A highly glazed extension can still become uncomfortable.

It simply means Part O's statutory regime is not generally the regulatory mechanism for an ordinary extension to an existing house.

Part H — Drainage

Drainage is often underestimated during extension design.

A double-storey extension may affect:

foul drainage;

rainwater drainage;

existing inspection chambers;

below-ground pipes.

Approved Document H covers foul and rainwater drainage and matters including pipework and inspection chambers.

Existing Drain Runs Under the Extension

This needs to be identified early.

The project may require:

drainage diversion;

protection;

redesign;

build-over arrangements where relevant.

Do not wait until the foundations are excavated to discover an inspection chamber directly beneath the proposed wall.

Foul Drainage

A new:

WC;

basin;

shower;

bath;

kitchen

may require new or altered foul-water connections.

The drainage route should be shown where relevant.

Rainwater Drainage

The enlarged roof increases rainwater collection.

The design may need:

gutters;

downpipes;

soakaway;

surface-water drainage connection

depending on site conditions.

Soakaways

A soakaway is not automatically suitable for every site.

Suitability depends on:

soil infiltration;

available space;

proximity to buildings;

drainage strategy.

Drainage and Foundations Must Be Coordinated

A drain running through:

a column foundation

is a design conflict.

Technical drawings should identify these interactions before work begins.

Part K — Stairs and Guarding

If the extension changes the staircase or introduces new steps, Part K may become relevant.

It addresses matters including:

stairs;

guarding;

protection from falling;

collision.

Does Every Double-Storey Extension Need a New Staircase?

No.

Many extensions simply expand the existing first-floor accommodation using the existing house staircase.

But if the project:

moves the stair;

extends it;

creates split levels;

the stair design needs to comply.

Headroom

Where stairs are altered, headroom needs proper consideration.

Do not design the floor plan first and only later discover that:

roof;

landing;

beam

occupies the required stair space.

Guarding

New landings, balconies or level changes may need guarding designed to the applicable standards.

Part P — Electrical Safety

New electrical work within the extension needs to comply with the electrical-safety requirements for dwellings.

This can include:

sockets;

lighting;

circuits;

kitchen electrical work.

Appropriate certification should be provided through the applicable compliance route.

Heating

The extension also affects the home's heating system.

Possible changes include:

new radiators;

underfloor heating;

heat-pump changes;

boiler-system alterations.

These need coordination with applicable energy requirements.

Plumbing

Where the first floor includes:

bathroom;

ensuite;

the design should consider:

hot water;

cold water;

waste pipes;

floor penetrations.

Services Through Structural Members

Never assume structural joists can simply be:

drilled;

notched;

cut

wherever plumbing or electrical contractors want.

Service zones should be coordinated with structural requirements.

Sound Insulation

A normal extension to a single dwelling is different from creating a new flat or separating dwellings.

However, internal acoustic quality can still be considered between:

bedrooms;

bathrooms;

living spaces.

Where the project involves a material change of use or separate dwelling, additional Part E requirements may apply.

Existing Openings Into the Extension

A double-storey extension may remove a significant area of the existing rear elevation.

That can affect:

structural stability;

support;

energy performance;

fire layout.

The opening should be designed as part of the whole building rather than treated as an isolated steel.

Do I Need Structural Calculations?

For many double-storey extensions:

yes, for the structural elements that require engineering design.

This may include:

rear opening beam;

floor beams;

columns;

roof beams;

frames;

unusual foundations.

Does Every Part of the Extension Need a Calculation?

No.

A conventional element may sometimes use recognised prescriptive guidance.

Structural calculations should be prepared where needed for the actual structural design.

What Might the Structural Calculation Package Include?

For example:

B1

Ground-floor rear opening.

B2

First-floor beam.

J1

First-floor joists.

C1 / C2

Steel columns.

F1 / F2

Column foundations.

R1

Roof beam.

The actual schedule depends entirely on the design.

Do I Legally Need a Structural Engineer?

Building Regulations do not simply state that every double-storey extension must appoint a person using the title:

Structural Engineer.

The design must, however:

comply with Part A;

be undertaken by people competent for their work.

A double-storey extension often contains enough structural complexity that specialist structural-design input is appropriate.

Structural Designer vs Principal Designer

These are different roles.

A person preparing structural calculations is a:

designer

for that structural scope.

They are not automatically the project's:

Building Regulations Principal Designer.

The Principal Designer has wider duties involving coordination of design work.

Building Regulations Dutyholders

For building work in England, the Building Regulations dutyholder regime can include:

client;

designers;

Principal Designer where more than one designer is involved;

contractors;

Principal Contractor where more than one contractor is involved.

These are legal roles under the Building Regulations framework.

They should not be confused with equivalent-sounding roles under CDM 2015.

Does BuildRegs Automatically Become Principal Designer?

No.

Providing:

Building Regulation drawings;

structural calculations

does not automatically mean BuildRegs has accepted appointment as the statutory Building Regulations Principal Designer.

The dutyholder appointments and scope should be made clear separately.

Full Plans or Building Notice?

For a double-storey extension, Full Plans is generally the more controlled route where a detailed technical package is available.

Full Plans

Current government guidance describes Full Plans as the most thorough application option.

A formal decision can normally be expected within:

5 weeks

or:

2 months with your consent.

Advantages of Full Plans

For a double-storey extension, it allows the technical design to be assessed before too much construction is committed.

That can be valuable because the project may contain:

foundations;

several structural elements;

energy details;

drainage;

fire-safety information.

Building Notice

Building Notice is available only for suitable smaller projects.

Current government guidance says work can generally start:

2 days

after the notice has been submitted.

However:

there is no formal approval of plans.

Building Control can still request information such as:

structural calculations;

technical details.

Is Building Notice Better Because It Is Faster?

Not necessarily.

It can allow work to begin sooner procedurally.

But a double-storey extension still needs:

technical design;

structural design;

compliant construction.

Starting without enough design information can shift uncertainty onto site.

Full Plans Is Often the More Sensible Route for a Double-Storey Extension

Because there are more interacting technical issues than in many smaller alterations.

A coordinated Full Plans package can reduce uncertainty before:

foundations;

steel orders;

first-floor construction;

roof works.

Can Work Start Before Full Plans Approval?

For ordinary non-higher-risk local-authority work, current government material indicates work may in relevant cases begin after the applicable notice period rather than waiting for the final five-week decision.

But proceeding before approval is:

at the client's risk.

If Building Control later requires changes, work already constructed may need alteration.

For a substantial double-storey extension, obtaining technical certainty first is often prudent.

Local Authority or Registered Building Control Approver?

For ordinary projects in England, Building Control can generally be provided by either:

local-authority Building Control;

registered building control approver.

Fees vary.

Local authorities base charges on their costs, while private registered approvers negotiate their fees directly.

Building Control Fees

Building Control fees are separate from:

architectural fees;

structural calculations;

Building Regulation drawings.

They vary according to:

project;

floor area;

inspection requirements;

Building Control provider.

What Does Building Control Inspect?

The exact inspection plan varies, but extension inspections can include stages such as:

excavation;

foundations;

drainage;

damp-proofing;

structural work;

insulation;

roof;

completion.

Follow the inspection schedule agreed with the Building Control body.

Do Not Cover Work Before Required Inspection

For example:

foundations;

drains;

insulation;

structural connections

may need to remain visible until inspected where required.

Ask the Building Control body rather than assuming.

Foundations Inspection

This commonly occurs after excavation and before concrete.

If Building Control needs:

deeper foundations;

wider foundations;

engineering information,

it is far easier to address before the concrete is poured.

Structural Steel Inspection

Building Control may want to inspect or receive information concerning:

steelwork;

bearings;

fire protection;

structural calculations.

The relevant details should match the approved/current design.

Completion

At the end of the project, Building Control needs the relevant completion information and inspections before issuing a completion certificate where the requirements are satisfied.

This is distinct from:

planning completion

or simply:

the builder finishing work.

Why a Completion Certificate Matters

It provides evidence that the Building Control process has been completed.

This can become important when:

selling;

remortgaging;

demonstrating compliance.

Party Wall etc. Act 1996

A double-storey extension can also fall within the scope of the:

Party Wall etc. Act 1996

depending on the work.

Potential triggers can involve:

work to a party structure;

excavation near neighbouring structures;

construction close to a boundary.

This is separate from:

planning;

Building Regulations.

Does Building Control Deal With Party Wall Matters?

No.

Building Control approval does not remove obligations under the Party Wall etc. Act.

Planning, Building Regulations and Party Wall Are Three Separate Issues

A project may therefore need:

Planning

↓

Building Regulations

↓

Party Wall procedures

depending on the circumstances.

Common Double-Storey Extension Mistakes

1. Treating Planning Drawings as Construction Drawings

They usually do not contain enough technical information.

2. Starting Foundations Before the Design Is Resolved

Ground conditions or structural loads can then create expensive changes.

3. Ordering Steel Before Calculations

The structural design should come first.

4. Ignoring Existing Structure

Loads may transfer back into old walls, beams or foundations.

5. Designing the Kitchen Before Columns Are Known

A column can end up where the kitchen designer placed an appliance.

6. Forgetting Drainage

An inspection chamber can clash directly with new foundations.

7. Over-Glazing Without Checking Part L

Large bifolds plus first-floor glazing can trigger additional energy calculations.

8. Changing Insulation on Site

Different products do not automatically have equivalent build-ups.

9. Ignoring Fire Strategy

Especially where a large open-plan ground floor changes escape from the first floor.

10. Leaving Structural Design Until the Builder Starts

That turns design questions into site delays.

The Better Sequence

1. Planning Design

Establish the proposed extension.

↓

2. Planning Permission / Confirm Permitted Development

As applicable.

↓

3. Building Regulation Drawings

Develop the technical construction.

↓

4. Structural Calculations

Design beams, joists, columns and relevant foundations.

↓

5. Coordinate Drawings + Calculations

Resolve clashes.

↓

6. Building Control

Submit the technical package.

↓

7. Builder Pricing

Price against a defined specification.

↓

8. Construction + Inspections

Build the coordinated design.

Why Structural Calculations Should Be Done Before the Final Builder Quote

Imagine the tender drawing says:

Large opening — steel beam required.

Builder A assumes:

one beam.

Builder B assumes:

beam + columns.

Builder C assumes:

frame + pad foundations.

Those are not comparable quotations.

Design First. Price Second.

Once the technical package identifies:

B1

main beam.

C1/C2

columns.

F1/F2

foundations.

contractors can quote a much more defined structural scope.

What Information Does BuildRegs Need?

If you already have drawings, upload them.

Useful files include:

existing plans;

proposed plans;

elevations;

sections;

roof plan;

planning decision drawings;

PDFs;

DWGs;

photographs;

Building Control comments.

What If I Don't Have Building Regulation Drawings Yet?

That is fine.

The existing planning or architectural set can often be used as the starting point for the technical work.

What If I Have No DWG?

Upload the PDF.

DWG is useful, but it is not a universal prerequisite.

Do I Need a New Measured Survey?

Not automatically if suitable reliable drawings already exist.

The project can be reviewed first.

Can BuildRegs Work Entirely Online?

For suitable extension projects, the technical design can often be developed remotely from reliable:

drawings;

measurements;

photographs;

project information.

If physical investigation is required, that should be identified rather than guessed.

What If Foundation Information Is Unknown?

Do not invent it.

The designer or Building Control may request:

trial hole;

site information;

ground information

where needed.

How Long Do the Drawings and Calculations Take?

For suitable BuildRegs projects, the typical technical turnaround is around:

7 working days

once the information required for the agreed scope has been received.

Complex projects or projects requiring investigation may take longer.

How Much Do Double-Storey Extension Building Regulation Drawings Cost?

BuildRegs currently advertises:

Double-Storey Extension

Building Regulation Drawings + Structural Calculations From £999

The final fixed quotation is confirmed after the project scope is reviewed.

What Can Affect the Price?

Examples include:

multiple structural openings;

steel frames;

complex roofs;

unusual foundations;

several structural calculations;

major design changes;

incomplete drawings.

Is Building Control Included in the £999?

No, unless explicitly stated in the confirmed quotation.

Building Control charges are separate.

Is Planning Included?

No, unless separately agreed.

Article 47 relates to the technical Building Regulations stage.

Does £999 Apply to Every Double-Storey Extension?

No.

It is a:

starting price.

A straightforward extension is different from a project with:

multiple beams;

extensive steel framing;

difficult foundations;

complex roof design.

The project is reviewed before the fixed price is confirmed.

What Does the BuildRegs Package Aim to Provide?

For the agreed scope, the technical package can combine:

Building Regulation Drawings

with:

Structural Calculations

so that the structural design and technical construction information are coordinated.

Why Combine the Two?

Because:

Drawing

may show where:

B1

goes.

The:

Calculation

determines why:

B1

works.

The construction package needs both to agree.

Example Double-Storey Extension Structural Schedule

A project might contain:

Ref	Element
B1	Ground-floor rear opening beam
B2	First-floor structural beam
J1	First-floor joists
C1	Left column
C2	Right column
F1	Left column foundation
F2	Right column foundation
R1	Roof member

Not every project needs all of these.

Frequently Asked Questions