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Preparing a house extension project for planning submission

Preparing to Build Your House Extension

Practical guidance to help you organise your project, compare quotations properly and avoid unnecessary problems before building work begins.

Once the design has been agreed and the necessary approvals are underway, attention turns to the next important stage: preparing for construction.

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This is where careful planning can make a significant difference.

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Starting too quickly, appointing a builder before the drawings are ready or relying on an incomplete quotation can lead to misunderstandings, delays and unexpected costs later.

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A successful extension project usually begins with a clear scope of work, suitable drawings, realistic quotations and a sensible understanding of who will be responsible for each part of the build.

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Make Sure the Design Is Clear

 

Before asking builders to provide firm quotations, the proposed extension should be clearly defined.

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The builder needs to understand:

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  • the size and shape of the extension;

  • the roof design;

  • the location of windows, doors and rooflights;

  • any internal walls to be removed or altered;

  • the proposed kitchen or bathroom layout;

  • structural openings and steelwork;

  • drainage requirements;

  • the expected finishes;

  • any work required within the existing house.

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A basic sketch may be enough for an initial conversation, but it is rarely sufficient for a meaningful quotation.

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The clearer the information provided at the outset, the easier it is to compare builders fairly and avoid important items being overlooked.

Obtain the Right Drawings​

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The drawings used for a planning application are not always detailed enough for construction.

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Planning drawings generally show the scale, appearance and basic layout of the proposal. Building Regulations drawings go further and explain the technical requirements of the build.

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Depending on the project, these may include details relating to:

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  • foundations;

  • walls and insulation;

  • roof construction;

  • structural openings;

  • ventilation;

  • drainage;

  • fire safety;

  • energy performance;

  • glazing;

  • floor construction;

  • damp protection;

  • electrical and plumbing requirements.

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Structural calculations may also be required for steel beams, roof structures, supporting walls and foundations.

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A builder can price and construct the project more accurately when the technical information has been prepared in advance.

Decide What You Want the Builder to Price

 

Not every homeowner wants the same type of building contract.

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Some prefer a builder to manage the entire project from start to finish. Others want a quotation for the main structural and construction work while obtaining separate prices for items such as glazing, kitchens, flooring or decorating.

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Before requesting quotations, decide whether you want the builder to include:

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  • demolition and site preparation;

  • foundations and groundworks;

  • drainage alterations;

  • structural steelwork;

  • walls, insulation and roofing;

  • windows, doors and rooflights;

  • electrical work;

  • plumbing and heating;

  • plastering;

  • decorating;

  • flooring;

  • kitchens and appliances;

  • bathrooms;

  • landscaping and external works;

  • waste removal and skips.

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If these items are not clearly identified, one builder may include them while another leaves them out. This can make a lower quotation appear more attractive than it really is.

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Compare Builder Quotations Properly

 

A builder’s quotation should be detailed enough for you to understand what is included and what remains uncertain.

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Do not compare quotations based on the final figure alone.

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A useful quotation should explain the main elements of the work and identify any exclusions, provisional sums or allowances.

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Look carefully for differences in:

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  • the scope of demolition;

  • foundation assumptions;

  • structural-steel allowances;

  • drainage work;

  • roof construction;

  • glazing;

  • electrical and plumbing allowances;

  • kitchen and bathroom installation;

  • internal finishes;

  • decoration;

  • VAT;

  • waste removal;

  • scaffolding;

  • external works.

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The cheapest quotation is not always the best value. A detailed and realistic quotation from an experienced builder may be safer than a low headline price with important items missing.

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[Read More: How to Compare Builder Quotations Properly]

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Check the Builder’s Experience

 

House extensions can be more complicated than they first appear.

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Unlike a new-build project, the builder must connect new work to an existing property while managing access, drainage, structural alterations, weather protection and the day-to-day impact on the household.

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Ask prospective builders about similar projects they have completed.

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Useful questions include:

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  • Have you completed extensions of this type before?

  • Can you provide examples of recent work?

  • Are references available?

  • Who will supervise the site?

  • Will the same team remain on the project?

  • How long do you expect the work to take?

  • How will changes or additional work be priced?

  • What insurance cover do you hold?

  • Are you comfortable working from detailed drawings and structural calculations?

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A good builder should be willing to discuss the project openly and explain how the work will be organised.

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Ask for References

 

References are useful, but they should be relevant.

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A builder may be perfectly capable of completing small maintenance jobs but have limited experience of larger extensions involving steelwork, drainage and major internal alterations.

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Ask to speak with previous clients who have completed projects of a similar scale.

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Where possible, ask about:

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  • communication;

  • reliability;

  • site organisation;

  • workmanship;

  • how problems were handled;

  • whether the final cost was reasonably close to the original quotation;

  • whether the project finished within a realistic timescale;

  • whether the homeowner would use the builder again.

Understand the Payment Structure

 

A clear payment structure protects both the homeowner and the builder.

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Avoid paying a large proportion of the construction cost upfront without a clear reason.

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Payments are often linked to progress, such as:

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  • an agreed deposit;

  • completion of foundations;

  • walls reaching roof level;

  • roof completion;

  • installation of windows and doors;

  • first-fix services;

  • plastering;

  • second-fix work;

  • final completion.

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The precise structure will vary depending on the size of the project and the builder’s terms.

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The important point is that payments should be clearly agreed, documented and linked to identifiable stages of work.

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Consider a Written Building Contract

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A written contract can help prevent misunderstandings.

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It should set out:

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  • the agreed price;

  • the scope of work;

  • the drawings and documents forming part of the agreement;

  • the payment schedule;

  • the anticipated start date;

  • the expected programme;

  • how variations will be handled;

  • who is responsible for ordering key materials;

  • insurance arrangements;

  • how disputes will be managed;

  • the position if the project is delayed.

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Even where the builder has been recommended, it is sensible to record the agreement properly.

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Allow for a Contingency Fund

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Unexpected issues can arise during almost any extension project.

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Once work begins, the builder may discover drainage problems, defective existing walls, hidden pipework, poor ground conditions or other issues that could not reasonably have been identified beforehand.

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It is sensible to retain a contingency allowance.

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As a broad starting point, many homeowners allow approximately 10–15% of the expected build cost for unforeseen work and changes.

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A contingency is not an invitation to spend more than necessary. It is a buffer that helps prevent the project from becoming financially stressful if something unexpected occurs.

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Consider the Practical Impact on Your Home

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If a key item arrives late, it can delay other trades and extend the overall programme.

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Before construction begins, ask the builder which items need to be selected and ordered in advance.

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Check Whether Party Wall Procedures Apply

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Some extension projects may require notices under the Party Wall etc. Act 1996.

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This can become relevant where the works involve:

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excavating foundations near a neighbouring property;

building on or close to a boundary;

cutting into a party wall;

removing chimney breasts;

carrying out structural work to shared walls.

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Where Party Wall procedures apply, they should be addressed early enough to avoid delaying the proposed start date.

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[Read More: Do I Need a Party Wall Agreement for My Extension?]

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Building Control and Site Inspections

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Most extensions require Building Regulations approval.

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During construction, inspections are normally required at key stages of the work. These may include foundations, drainage, insulation, structural elements and final completion.

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The builder should understand the inspection process and allow for it within the programme.

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Do not allow important stages of the work to be covered over before the relevant inspection has taken place.

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Keep a Record of Decisions

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Changes are common during building projects.

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A homeowner may decide to move a window, alter the lighting arrangement, add a rooflight or upgrade a finish once the work is underway.

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Keep a written record of any changes, including:

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what has changed;

the additional cost;

any impact on the programme;

whether revised drawings are required;

whether the change affects approvals or structural design.

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A short written confirmation can prevent misunderstandings later.

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A Good Start Makes a Better Project

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The time spent preparing for construction is rarely wasted.

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Clear drawings, realistic quotations, a suitable builder and an agreed scope of work provide the foundation for a smoother build.

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No project is completely free from surprises, but good preparation helps reduce unnecessary risks and gives homeowners far more control over the cost, quality and progress of the work.

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Getting Ready to Build?

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Tell us a little about your extension, your location and the stage you have reached.

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Where appropriate, APN Extensions can help you request introductions to suitable extension professionals, including builders, architectural-design professionals, structural engineers, Building Control professionals, Party Wall surveyors, glazing specialists and project managers.

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