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Practical advice for planning a residential extension

What Drawings Do Builders Need Before Quoting?

A builder can provide an early budget estimate from a basic design, but a reliable quotation requires far more detail. The clearer the drawings and specification, the easier it is to compare prices properly and avoid expensive surprises later.

Once you have decided to extend your home, it is tempting to contact several builders immediately and ask for prices.

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This is understandable. Most homeowners want to know whether the project is affordable before committing too much time and money to the next stage.

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However, there is an important difference between an early budget estimate and a detailed quotation.

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A builder may be able to provide an approximate cost range after looking at a few plans, photographs or a simple description of the project. But if you want a meaningful quotation that can be compared fairly with other prices, the builder will normally need a much clearer package of information.

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The quality of the quotation depends heavily on the quality of the information provided.

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Planning Drawings Are a Useful Starting Point

 

Planning drawings usually show the overall shape, size and appearance of the proposed extension.

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They may include:

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  • Existing and proposed floor plans

  • Existing and proposed elevations

  • Roof plans

  • Site or block plans

  • Basic dimensions

  • The position of doors, windows and rooflights

  • The relationship between the extension and the existing house

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These drawings are valuable because they help a builder understand the general scale of the work.

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For a simple project, an experienced builder may be able to use planning drawings to provide a broad estimate or provisional price range.

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However, planning drawings are not usually intended to explain every construction detail. They may not show how the extension is to be built, what structural work is required or which materials and finishes should be included in the quotation.

Why Planning Drawings Alone May Not Be Enough

 

Two extensions can look almost identical on a planning drawing but have very different construction costs.

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One project might involve straightforward foundations, a simple roof and a modest opening into the existing house.

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Another could require deep foundations, complex drainage alterations, large steel beams, extensive temporary support, bespoke glazing and the removal of several load-bearing walls.

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If these details are unclear, each builder may make different assumptions.

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One builder may allow for basic finishes. Another may include higher-quality materials. A third may leave out important items entirely and describe them as extras later.

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This makes it difficult to compare quotations properly.

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The cheapest initial price is not necessarily the cheapest final project.

Building Regulations Drawings

 

Before requesting detailed quotations, it is usually sensible to prepare a Building Regulations drawing package.

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These drawings develop the approved or preferred design into a more detailed construction proposal. They help explain how the extension should be built and how the work is intended to satisfy the relevant technical requirements.

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Depending on the project, the drawings may include:

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  • Proposed floor plans with detailed dimensions

  • Foundation details

  • External wall construction

  • Floor construction

  • Roof structure and insulation build-up

  • Ceiling details

  • Drainage layouts

  • Ventilation requirements

  • Fire-safety considerations

  • Thermal-insulation information

  • Door and window positions

  • Rooflights and roof lanterns

  • Rainwater drainage

  • Notes relating to materials and workmanship

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For straightforward projects, a clear Building Regulations package gives builders a much stronger basis for pricing.

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It also helps reduce the number of assumptions hidden inside the quotation.

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Structural Drawings and Calculations

 

Many extensions require structural engineering input.

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This is particularly important where the project involves:

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  • Removing a load-bearing wall

  • Creating a wide opening between the original house and the extension

  • Installing steel beams

  • Supporting existing walls above new openings

  • Altering the roof structure

  • Building close to trees or poor ground conditions

  • Constructing a double-storey extension

  • Combining an extension with major internal alterations

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The structural engineer may provide calculations, beam sizes, padstone details, connection details and supporting drawings.

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Builders need this information because structural work can represent a significant part of the cost.

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A quotation based on guessed beam sizes or an assumed structural design is likely to change once the engineer’s information becomes available.

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Drainage Information

 

Drainage is one of the most common sources of unforeseen cost.

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An extension may be built over or close to existing drains, inspection chambers or shared drainage runs. Pipes may need to be diverted. New chambers may be required. The proposed kitchen, utility room or WC may also need new connections.

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Before pricing the work, the builder should understand:

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  • Where the existing drains are located

  • Whether any drains are shared

  • Whether an inspection chamber needs to be moved

  • Where new waste pipes will connect

  • Whether a build-over agreement may be relevant

  • How rainwater will be managed

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Some drainage details cannot be confirmed until the site is opened up. However, identifying the likely issues early allows the builder to include sensible allowances rather than ignoring the risk entirely.

A Written Specification


Drawings are important, but they cannot describe every product, finish or assumption.
 

A written specification helps explain what should be included in the price.
 

This may cover:
 

  • Bricks, blocks, render or cladding

  • Roof tiles, slate or membrane finishes

  • Insulation standards

  • Windows and external doors

  • Rooflights or roof lanterns

  • Internal doors

  • Plastering and decorating

  • Flooring

  • Electrical points and lighting

  • Plumbing and heating

  • Kitchen installation

  • Sanitary fittings

  • Drainage works

  • External paving and making good

  • Waste removal

  • Scaffolding

  • Building Control inspections

  • Guarantees and certification
     

The specification does not always need to name every final product at the earliest stage. However, it should be detailed enough for each builder to price a similar level of work.
 

Where the homeowner has not yet chosen a particular item, the quotation can include a realistic provisional allowance.

 

What Are Provisional Sums?


A provisional sum is an allowance for an item that cannot yet be priced precisely.
 

For example, a builder may include a provisional sum for drainage alterations if the exact depth and route of the existing pipes cannot be confirmed before excavation.
 

Provisional sums are not automatically a problem. Some uncertainty is unavoidable in building work, particularly when dealing with older homes.
 

The important point is that they should be clearly identified.
 

A quotation containing a number of transparent provisional allowances is usually more useful than an unrealistically low quotation that quietly excludes uncertain items and adds them later as extras.

 

Are Kitchen, Bathroom and Finishing Costs Included?


This is one of the areas most likely to cause confusion.
 

Some builders quote only for the structural shell of the extension. Others price for a completed room but exclude the kitchen units, appliances, worktops, flooring or decoration. Some will include plumbing and electrical connections but not the final fittings.
 

Before comparing prices, establish exactly what each quotation includes.
 

Ask whether the builder has allowed for:
 

  • Kitchen units and installation

  • Appliances

  • Worktops

  • Bathroom fittings

  • Tiling

  • Flooring

  • Painting and decorating

  • Light fittings

  • Underfloor heating

  • Radiators

  • External paving

  • Landscaping

  • Blinds or specialist glazing treatments
     

A lower quotation may simply include less work.

 

Shell-Only Quotations


Some homeowners deliberately request a shell-only quotation.
 

This can be useful where they plan to arrange the kitchen, flooring, decoration or specialist glazing separately.

A typical shell-only package might include the foundations, walls, roof, external doors, windows, structural steelwork and basic internal plaster finish.
 

However, there is no universal definition of a shell.
 

One builder’s shell price may include electrics and plumbing. Another may exclude them. One may include decoration. Another may stop at bare plaster.
 

Always ask the builder to set out clearly what is included and excluded.

 

Tendering to Several Builders


For a meaningful comparison, each builder should receive the same information.
 

Ideally, this should include:
 

  • The latest design drawings

  • Building Regulations drawings

  • Structural information where available

  • A written specification

  • Any relevant drainage information

  • A list of expected finishes

  • A request for clear inclusions and exclusions

  • A request for identified provisional sums

  • A proposed start date or approximate programme
     

If builders are pricing from different versions of the drawings or making different assumptions, the quotations will not be directly comparable.
 

It is often helpful to ask each builder to return a cost breakdown rather than one headline figure.

 

What Should a Builder’s Quotation Include?


A good quotation should be clear enough for you to understand what you are buying.
 

It may include separate allowances for:
 

  • Site setup

  • Demolition and strip-out works

  • Groundworks and foundations

  • Drainage

  • Brickwork and blockwork

  • Structural steelwork

  • Roofing

  • Windows, doors and glazing

  • Insulation

  • Plumbing and heating

  • Electrical works

  • Plastering

  • Carpentry

  • Decoration

  • Flooring

  • Kitchen or bathroom installation

  • External making good

  • Waste removal

  • Scaffolding

  • VAT
     

It should also identify exclusions, provisional sums and any assumptions made by the builder.
 

A detailed quotation makes it easier to identify significant differences between prices before signing a contract.

 

Should You Use the Cheapest Builder?


Price matters, but it should not be the only consideration.
 

A quotation that appears considerably cheaper than the others may contain omissions, unrealistic allowances or misunderstandings about the scope of work.
 

Before appointing a builder, consider:
 

  • The detail and clarity of the quotation

  • Relevant experience

  • References from previous clients

  • Similar completed projects

  • Insurance

  • Availability

  • Communication

  • Payment stages

  • The proposed programme

  • How changes and extras will be managed
     

A builder who has taken the time to understand the drawings and prepare a careful quotation may offer better value than someone who has provided a low headline figure with very little detail.

 

Avoiding Costly Surprises


No set of drawings can eliminate every unknown.
 

Older buildings may conceal defective drains, unexpected structural issues, poor ground conditions or previous alterations that only become visible once construction begins.
 

However, properly prepared drawings, structural information and a sensible written specification can remove much of the avoidable uncertainty.
 

They allow builders to price the same project on a broadly consistent basis and help homeowners make a more informed decision.

 

Prepare Properly Before Requesting Quotations


Before asking builders to provide detailed prices, make sure the project has moved beyond the basic idea stage.

A coordinated package of design drawings, Building Regulations details, structural information and specifications will usually produce more reliable quotations and reduce the likelihood of disputes later.
 

If you are planning an extension and need help preparing the drawings or connecting with suitable professionals, tell us a little about your project. We can help you explore the next steps and find the right support for the work ahead.

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