What Drawings Do I Need for a House Extension? A UK Homeowner’s Guide

If you are planning a house extension, you will usually need different drawings at different stages of the project. There is not one universal set of “extension plans” that performs every job.

For a typical extension, the drawing process may progress from an existing-property survey and design drawings, through planning or Permitted Development information, and then into Building Regulations and technical drawings. Structural calculations and details may also be required where the proposal involves structural alterations.

The important distinction is that each drawing package answers a different question:

Existing drawings — what is there now?

Proposed/design drawings — what do you want to change?

Planning drawings — what are you proposing for planning purposes?

Building Regulations/technical drawings — how is the work intended to satisfy applicable technical requirements and be constructed?

Structural information — how will relevant structural loads be safely supported?

Understanding these stages before appointing a builder can make an extension considerably easier to design, approve, price and construct.

Before designing an extension properly, you need an accurate understanding of the existing property.

For many residential projects, this begins with a measured survey.

The resulting existing drawings may include:

• existing floor plans;

• existing elevations;

• relevant building sections;

• roof information;

• doors and windows;

• external walls;

• visible drainage features;

• site boundaries and nearby structures where relevant; and

• other constraints affecting the proposed work.

The exact information required depends on the property and project.

Why do existing drawings matter?

An extension does not exist independently from the original house.

Its design may depend on:

• existing room dimensions;

• floor levels;

• window positions;

• roof geometry;

• structural walls;

• drainage;

• neighbouring buildings; and

• how people currently move through the house.

An inaccurate survey can therefore cause problems much later.

If an existing wall, opening or level has been recorded incorrectly, that error can propagate into the proposed design and technical drawings.

Good extension design begins with reliable existing information.

Once the existing house has been recorded, proposed drawings can explore how the property should change.

These drawings are not simply about deciding how many metres the extension projects into the garden.

A useful extension design should consider the existing house and proposed addition together.

For example:

Will the extension contain the kitchen?

Where will the dining table sit?

Should the existing rear wall remain partly in place or be substantially opened?

How will the extension affect daylight to the original rooms?

Where will furniture and storage go?

How will you move from the front door through the house to the garden?

Where should new windows, rooflights or glazed doors be positioned?

The proposed floor plan is therefore one of the most important design drawings in the project.

It is where the extension begins to become a home rather than simply additional floor area.

If your extension requires planning permission, the local planning authority needs sufficient information to understand and assess the proposed development.

Depending on the project and application requirements, the drawing package can include:

• location plan;

• site or block plan;

• existing floor plans;

• proposed floor plans;

• existing elevations;

• proposed elevations;

• roof plans where relevant;

• sections where required; and

• dimensions and other information necessary to describe the proposal.

Planning drawings principally communicate what development is proposed and how it relates to the existing property and surroundings.

They are not automatically construction drawings.

Not every extension requires a conventional planning application.

Some house extensions can potentially be carried out under Permitted Development rights where the property and proposal satisfy all relevant limitations and conditions.

But Permitted Development does not mean drawings have no value.

The proposed work still needs to be designed and checked against the applicable rules.

A homeowner may also choose to apply for a Lawful Development Certificate to obtain formal confirmation that proposed work is lawful for planning purposes.

And importantly:

Permitted Development does not remove the separate requirement to comply with Building Regulations where they apply.

Planning and Building Regulations are different systems.

Once the architectural design has been established, the project needs to move from:

“What are we building?”

towards:

“How is it going to work technically?”

This is the role of Building Regulations and technical design information.

For an extension, relevant information may address matters such as:

• foundations;

• ground-floor construction;

• external-wall construction;

• roof construction;

• thermal insulation;

• structural stability;

• fire safety;

• ventilation;

• drainage;

• windows and doors;

• electrical safety;

• moisture protection; and

• other applicable requirements.

The precise information depends on the individual project.

Current GOV.UK guidance describes Full Plans as the most thorough Building Regulations application option for work outside the higher-risk building regime. Under that route, plans and supporting information are checked before a formal decision is issued.

Local-authority guidance illustrates the level of information that can accompany Full Plans applications. Manchester City Council, for example, asks for detailed plans, sections and elevations incorporating necessary technical notes, supporting calculations and specifications.

That demonstrates an important point:

Planning drawings and Building Regulations drawings are not interchangeable.

A section cuts vertically through the proposed construction.

It can communicate information that is difficult or impossible to understand from a floor plan alone.

Depending on the project, a technical section might illustrate the relationship between:

• foundation;

• ground;

• floor construction;

• damp protection;

• external wall;

• insulation;

• roof;

• ceiling;

• windows and doors;

• existing house; and

• proposed extension.

The drawing can then be supported by technical notes, details, specifications and specialist design information as appropriate.

This is where architectural drawings begin to describe how different parts of the building connect.

Many extensions require structural-engineering input, although the extent depends on the proposal.

Common examples include:

• removing part of the original external wall;

• creating a wide opening between the house and extension;

• steel beams;

• columns;

• structural roof alterations;

• unusual foundation conditions; or

• other load-bearing alterations.

Building Control may require supporting structural calculations where structural work forms part of the proposal. Current council guidance explicitly identifies structural calculations as information that may accompany Full Plans submissions where structural changes are involved.

Architectural drawings and structural engineering do different jobs

Suppose you want to remove most of the original rear wall so that your existing kitchen opens into a new extension.

The architectural drawings establish:

• the desired opening;

• room layout;

• relationship between old and new;

• floor and ceiling relationships; and

• overall construction arrangement.

The structural engineer determines how the relevant loads should be supported.

The resulting structural design then needs to be coordinated with the architectural information.

This coordination is particularly important where beams, columns or supports affect ceiling levels, walls, doors, kitchen layouts or other architectural elements.

You should not automatically assume so.

Planning drawings are prepared principally to communicate the proposed development for planning purposes.

They may not explain:

• wall construction;

• insulation;

• floor build-up;

• roof build-up;

• structural design;

• drainage alterations;

• damp protection;

• construction junctions; or

• other technical requirements.

A builder pricing only from planning drawings may therefore need to make assumptions about what has not yet been specified.

Different builders may make different assumptions.

That can make quotations harder to compare.

Better information can produce better pricing information

Technical drawings cannot eliminate every uncertainty in construction — particularly when altering an existing building where concealed conditions may only become apparent once work starts.

But a coordinated technical package can substantially reduce ambiguity.

Instead of asking several contractors to interpret a basic planning drawing differently, you can give them a more consistent description of the intended work.

That can improve the quality of tendering and quotation comparisons.

Not necessarily in every case.

For relevant work in England outside the higher-risk regime, there are different Building Regulations application routes.

Full Plans

This involves submitting detailed information for assessment.

GOV.UK describes this as the most thorough option and says a decision can generally be expected within five weeks, or two months with the applicant's consent.

Building Notice

This is intended for smaller projects and does not provide the same formal pre-construction approval of detailed plans.

GOV.UK states that work can begin two days after a Building Notice is submitted, but you do not receive formal approval in the same way as under Full Plans.

Some types of work cannot use the Building Notice route.

The appropriate route should therefore be established for the individual project.

Why might you still want technical drawings?

Building Regulations approval is only one reason to prepare technical information.

Drawings can also help:

• coordinate the designer and structural engineer;

• explain the intended construction to contractors;

• obtain more meaningful quotations;

• reduce assumptions;

• coordinate junctions and dimensions; and

• provide a record of the intended design before work begins.

So the question should not simply be:

“What is the minimum I need to submit?”

It should also be:

“What information does this project need to be properly coordinated, priced and constructed?”

Ideally, sufficient design and technical information should exist before you ask contractors for a firm price.

This does not mean every conceivable construction decision must be resolved before speaking to a builder.

Early contractor input can be valuable.

But asking several builders to price an underdeveloped scheme creates a problem:

They may not be pricing the same thing.

One contractor may assume a particular roof construction.

Another may assume something different.

One may include drainage alterations.

Another may exclude them.

One may anticipate significant structural steelwork.

Another quotation may contain an allowance rather than a defined solution.

The apparent difference in price may therefore reflect differences in assumptions rather than genuine differences in cost.

Although every project differs, a well-organised domestic extension might progress broadly as follows:

Stage A — Existing information

Measured survey and existing drawings.

Stage B — Proposed design

Develop the extension layout and appearance.

Stage C — Planning strategy

Determine whether the proposal requires planning permission or may fall within another appropriate planning route.

Stage D — Planning / lawful-development information

Prepare the drawings and documents appropriate to that route.

Stage E — Technical design

Develop the construction information needed for the Building Regulations and the project itself.

Stage F — Structural design

Obtain structural-engineering calculations and details where required.

Stage G — Coordination

Incorporate and coordinate relevant architectural and specialist information before construction.

This sequence avoids trying to solve every technical detail before the basic design is settled, while also avoiding the opposite mistake of arriving on site with little more than planning drawings.

Rather than simply asking:

“How much are extension plans?”

ask the designer exactly what their service includes.

Useful questions include:

• Is a measured survey included?

• Are existing drawings included?

• How many proposed design revisions are included?

• Are planning drawings included?

• Is the planning application submission included?

• Are Building Regulations drawings included?

• Is the Building Control application included?

• Is structural engineering included or separate?

• Who coordinates the structural information?

• Are technical sections and details included?

• What information will ultimately be issued to the builder?

Two quotations described as “extension drawings” may provide very different levels of service.

Do I need drawings for a small extension?

The information required depends on the extension and approval route. Even a relatively small extension benefits from properly coordinated design information, and Building Regulations may still apply regardless of whether planning permission is required.

Do I need planning drawings if my extension is Permitted Development?

You may not require a conventional planning application, but drawings are still useful for designing and assessing the proposal. If applying for a Lawful Development Certificate, appropriate drawings will normally form part of the application.

Are Building Regulations drawings the same as construction drawings?

The terms are sometimes used interchangeably in domestic work, but their scope can differ. Building Regulations information demonstrates relevant regulatory compliance, while a more extensive construction package may contain additional information needed to define and coordinate the works. Establish exactly what your designer is providing.

Does my structural engineer prepare all the drawings?

Usually not. Structural engineers generally design the structural elements within their scope. Architectural drawings coordinate the wider building design, layout and construction information. The two need to work together.

Can I get builders' quotes before Building Regulations drawings?

You can obtain early budget estimates, but detailed quotations are more meaningful when contractors have sufficiently developed and consistent information to price.

Does Building Control design the extension for me?

No. Building Control assesses applicable work against the Building Regulations. It should not be treated as the architectural or structural designer for your project.

Need Drawings for Your House Extension?

A successful extension normally progresses through more than one drawing stage.

Architectural Drawing Studio can assist with existing and proposed extension drawings, planning drawings and technical Building Regulations information, with structural-engineering information coordinated where required within the agreed project scope.

The objective is not simply to produce a plan that obtains an approval.

It is to develop clear architectural information that helps take your project from the existing house, through design and approvals, towards something a contractor can properly understand and build.