Planning an extension: questions to ask builders

Extending your home is a major project, and the way you prepare before contacting builders sets the tone for the whole process. Knowing what documents, permissions and professional advice you need in advance can make discussions with builders clearer, faster and less prone to misunderstandings. This guide helps you get ready to request quotes, understand who should be involved, and ask the right questions to compare local builders for your extension.

Prepare your drawings and approvals before seeking quotes

Accurate, detailed information is the foundation of any building quote. Builders need more than a sketch or a verbal idea to give you a meaningful price or programme. Before you approach builders:

• Have drawings ready: At a minimum, you will need scaled plans showing the layout, elevations and a clear description of what is to be built. These drawings are usually prepared by an architect or architectural technologist. They help builders understand the full scope and avoid confusion about what is included.
• Check your planning position: Some extensions fall within permitted development rights in England, provided they meet certain limits. Others require planning permission from your local planning authority. Even if you believe your extension is permitted development, consider obtaining written confirmation from the council. Builders will want to know your planning status before quoting.
• Structural calculations: For any extension involving changes to the structure—such as removing walls or building over multiple storeys—you will need calculations from a structural engineer. Having these ready means builders can price accurately for steelwork or changes to foundations.
• Building Regulations: Approval under Building Regulations is separate from planning permission. It covers structure, insulation, drainage and safety. You can go via the local authority or an approved inspector, and the route you choose may affect the builder’s programme and responsibilities.

Having these documents in place means you can provide the same information to each builder, making quotes easier to compare and reducing the risk of surprises later.

Decide how you want to run the design and build process

Extensions involve several professionals, and the way you organise the design and build process has real effects on cost, responsibility and communication. Think about:

• Architect or architectural technologist: These professionals prepare the design, drawings and planning documentation. Some will also manage the project or recommend builders they have worked with before. You can find architects near you and compare their services.
• Structural engineer: Their role is to check the structural safety of the extension and provide calculations for foundations, beams and any load-bearing changes.
• Builder: The builder carries out the physical work and often organises specialist trades such as electricians, plumbers and roofers. On Annuvell Local, you can compare builders who list extensions as a service at builders near you.
• Design-and-build firms versus separate design: Some builders offer a design-and-build service, handling drawings, permissions and construction as a single package. Others expect you to come with drawings and approvals in hand. Each approach has its pros and cons—design-and-build can be simpler for communication and responsibility, but separating design and build can offer more flexibility and transparency.

You do not need to decide everything before your first enquiry, but knowing which professionals you want to involve helps you ask the right questions from the start.

Builder questions specific to extensions

Extensions bring specific technical and practical issues that are less common in other building work. When comparing builders or discussing your plans, ask about:

• Foundations and unknown ground: Extensions often need new foundations, and the ground conditions are not always clear until work starts. Ask builders how they allow for possible changes if they hit unexpected ground or obstructions. A good answer sets out what is included in the price and what happens if extra work is needed.
• Drainage: Extending can mean relocating or connecting to existing drains and services. Ask how the builder would handle drainage changes, and whether they will liaise with Building Control or the water company if needed.
• Party wall considerations: If your extension is near a boundary or attached to a neighbour’s property, ask how they handle party wall matters. While builders are not responsible for serving notices, they should be familiar with the process and able to work with surveyors where required.
• Weatherproofing sequence: For extensions that open up to your existing house, find out how the builder sequences the work to keep your home dry and secure. This might involve temporary walls, roofing or staged openings.
• Living on site: Most extensions are built while the owners remain in the house. Ask about site safety, dust management, toilet facilities for the workforce, and how the site will be kept tidy. Clear answers help you plan for the disruption and set expectations on both sides.

You can compare builders’ experience with extensions and other relevant facts side by side using the Annuvell Local compare tool, making it easier to pick out those with the right background for your project.

Make sure the programme is realistic

The timeline for an extension depends on far more than just the size of the job. Weather, material lead times and the availability of specialist trades can affect when work starts and finishes. When you discuss the programme with builders:

• Ask how weather could affect the timing: Foundations and external work may be delayed by heavy rain or frost. Builders should explain how they allow for this in their programme, and how delays are handled if they occur.
• Check material lead times: Some materials, such as windows, doors or specialist steels, can have long delivery times. Ask when these need to be ordered and how the builder plans for this, to avoid delays once the job starts.
• Assess the builder’s availability: Good builders are often booked months ahead, especially for extensions. Ask when they could realistically start and how long similar jobs have taken in the past.
• Discuss how changes or extras are handled: Variations to the scope can add time. Make sure the builder will notify you of any changes and agree new timings before extra work begins.

A realistic programme, agreed up front, helps manage expectations and reduces stress if there are delays. For more on comparing quotes and what to expect, see what should be included in a building quote.

Payment structure and stage payments

Paying for building work in stages is common practice for extensions. A clear payment structure protects both you and the builder, and reduces the risk of disputes. Discuss with each builder:

• Stage payments: Typical stages include completion of foundations, the shell (walls and roof), making watertight (windows and doors in), first fix (initial wiring and plumbing), second fix (fitting out, plastering) and completion. Ask for a written schedule showing what work each payment covers.
• Deposits: Ask whether a deposit is required and what it covers. Some builders request a small deposit to secure a start date or order materials.
• How payments are requested: Payments should only be due when a stage is complete. Ask how the builder confirms completion of each stage and what evidence or sign-off you will see.
• Final payment: The last payment should be made only after the work is fully complete and any snags are addressed.
• Consumer rights: Under the Consumer Rights Act 2015, services must be carried out with reasonable care and skill, within a reasonable time and for a reasonable price if none was agreed. Contracts agreed at home, by phone or online usually have a 14-day cooling-off period, but if you ask for work to start within this period you can be charged for anything done.

A builder who is clear and open about payments and stages is easier to work with and less likely to spring surprises. For more on finding reputable traders, see finding a reputable builder for an extension or renovation.

Questions to ask before agreeing a contract

Before you sign anything or agree to start work, clarify key points with each builder you are considering:

• What exactly does the quote include? Make sure all works, materials, permissions, waste removal and any specialist trades are listed.
• Who will be on site? Will the builder use their own staff or subcontractors? Who will supervise the work day-to-day?
• How will you communicate? Agree how updates, issues and variations will be discussed and recorded.
• What insurance do you hold? Builders should have public liability insurance and, if using subcontractors, check their cover too.
• What happens if you or the builder want to change the programme or scope? A clear process for changes helps prevent disputes.

Writing down the answers and comparing them between builders helps you spot differences and choose the one who is easiest to deal with, not just the cheapest.

Next steps

Get your drawings, planning status and structural calculations in order before you approach any builders. Then find builders and architects near you, request quotes using the same information for each, and compare their responses on experience, approach, programme and payment structure. If you are not sure what should be in a quote or how to compare them, check what should be included in a building quote and finding a reputable builder for an extension or renovation for more detailed advice.