What an extension contract must include before you commit

A contract is the only thing standing between you and a project that spirals into chaos or conflict. Before you sign anything, you need to know exactly what you're paying for, when it will be done, and what happens if either party changes their mind.

The biggest mistake homeowners make is treating a contract like a formality. It's not. A vague agreement or a handshake deal is how you end up in a dispute over scope creep, payment schedules, or unfinished work with no legal recourse.

The scope of work must be specific, not vague

The contract needs to describe every element of the extension in detail. This means dimensions, materials, finishes, and exactly what is included and what is not. "Build a bedroom" is not enough. "Build a bedroom 4m × 3.5m with brick walls, plaster and paint finish, single-glazed windows, laminate flooring, and standard door frame" is the level of clarity you need.

Ask the builder to attach plans—even rough sketches—to the contract. These plans should show the footprint, any structural changes, roof line, and placement of openings. If the builder hasn't provided plans, that's a red flag. You cannot price or schedule work you cannot visualise, and neither can they.

The contract must also state whether services like project management, obtaining council approvals, or design input are included, or whether you are organising these separately. A misunderstanding here often leads to delays and blame.

Payment terms and schedule must be clear and realistic

Never agree to pay the full amount upfront. The contract should outline a payment schedule tied to milestones or stages of work. For example: deposit on contract signature, payment at foundation stage, payment at roof stage, and final payment on completion. This protects you because the builder remains motivated to move forward, and you retain leverage if work quality is poor.

The contract must specify the total contract price, what that price includes, and what triggers additional charges. Extras—changes you request after signing, or unforeseen conditions like underground obstacles—should require a written change order with a new price agreed in advance. Without this clause, you have no protection against surprise costs.

The contract should also state the timeline clearly. When does work start? What is the expected completion date? Are there penalties or adjustments if the builder delays? If the client requests changes that delay completion, this should be documented separately. A realistic timeline protects both parties because it sets expectations and creates accountability.

Warranties, insurance, and liability matter

The contract must confirm that the builder carries public liability insurance and that cover is current and relevant to the scope of work. Ask to see the certificate before you sign. This protects you if someone is injured on the site or if neighbouring property is damaged.

The contract should include a warranty period—typically 12 months from practical completion—during which the builder will fix defects caused by poor workmanship or materials. This is standard practice. The contract should also clarify whether the builder is responsible for inspections or whether you are arranging and paying for inspections separately.

If structural work is involved, the contract must confirm that a registered professional—an engineer or architect—is overseeing the work, and that compliance certificates will be provided at the end. Ask what happens if the builder finds problems during construction that change the scope or cost. This clause protects you from nasty surprises after demolition.

The small clauses that prevent big arguments

Make sure the contract covers site cleanliness, safety, and access. Who is responsible for securing the site? Who pays for debris removal? What happens if the builder needs access to your neighbours' property, and how will this be managed?

The contract should state what happens if work is suspended due to weather, supply shortages, or load-shedding. A reasonable clause might allow the builder to extend the timeline without penalty if delays are beyond their control, but payment schedules should be paused accordingly.

Finally, the contract must be signed by both parties and kept by both. You should receive a copy. If the builder resists putting terms in writing, do not proceed. A professional will welcome a clear contract because it protects them too.

When you're ready to compare builders and their contract terms side by side, Strove lets you collect quotes and review contractor profiles to make sure you're working with someone experienced in extensions who can articulate their process clearly.