A house extension quotation can look reassuringly simple: a total figure, a start date and a promise of more space. The detail underneath is what protects your budget. Use this extension quotation checklist before accepting a price, particularly where several trades, structural work and interior finishes need to come together.
A good quotation should make the project easier to understand. It should show what will be built, who is responsible for each part and where costs may change. If a quote leaves you guessing, comparing it fairly with another contractor becomes difficult.
Start with the same scope for every quotation
Before comparing prices, make sure each builder has priced the same information. Plans, structural drawings, building regulations requirements and a written description of your priorities should be shared with everyone quoting. This prevents one contractor allowing for a basic rear extension while another has included rooflights, underfloor heating and a finished kitchen area.
If plans are still being developed, a site-based estimate can be useful, but it should be treated as an early budget figure rather than a fixed commitment. The more decisions that remain open, the more allowances and assumptions a contractor will need to make.
Ask the contractor to describe the extension in plain terms. This should cover its footprint, wall construction, roof type, glazing, access arrangements and intended internal layout. For example, a single-storey kitchen extension with a flat roof is not a complete scope on its own. Details such as drainage alterations, steelwork, roof lanterns and the connection to the existing house can affect both cost and programme.
Extension quotation checklist: what should be included
A detailed quotation does not need to be filled with unnecessary technical language. It does need to be specific enough that you can see what you are paying for. Work through the following areas with the contractor.
Design, approvals and professional input
Confirm whether the quote includes planning support, building regulations submissions, structural engineer calculations and inspections. Some projects can proceed under permitted development, but that does not remove the need to consider building regulations or structural requirements.
It is also worth clarifying who will deal with party wall matters where applicable, and whether fees for surveys, applications or third-party professionals are included or excluded. These costs are often legitimate exclusions, but they should be visible from the outset.
Groundworks and structural work
The ground is one of the main reasons extension costs can change after work begins. Check whether the quotation allows for excavation, foundations, concrete, drainage runs, removal of spoil and making good the surrounding ground. Ask what happens if unsuitable ground, buried services or unexpected drainage issues are found.
For extensions that open into the existing home, confirm the steelwork and structural alterations included. The quote should identify whether it covers supply and installation of beams, temporary support, padstones, associated labour and any required fire protection. A low figure can be misleading if major structural items have only been mentioned generally.
The external envelope
Your extension needs to be weatherproof, insulated and properly connected to the existing property. Check the specification for walls, roof covering, insulation, damp-proofing, guttering, fascias, windows, external doors and brick or render finish.
Do not assume that a phrase such as “windows included” tells you enough. Ask about the number of units, opening styles, frame material, glazing specification, handles and whether trickle vents are required. The same principle applies to bi-fold doors, sliding doors and rooflights, where product choices can create a substantial price difference.
Internal finishes and essential trades
This is where quotations most often differ. One price may stop at plastered walls and a screeded floor, while another includes electrics, plumbing, heating connections, decorating and finished flooring. Neither approach is automatically wrong, provided it is clear.
Confirm the allowance for sockets, switches, lighting points, extractor fans, radiators or underfloor heating, plumbing feeds and waste pipes. If you are extending a kitchen or adding an en-suite, establish whether the quote includes first-fix and second-fix work, and whether sanitaryware, kitchen units and appliances are supplied, fitted or excluded.
Ask about plastering, skirting boards, internal doors, tiling, flooring and decoration too. These are the details that turn a completed structure into a usable room.
Site management and protection of your home
A well-managed project accounts for the practical reality of building work around an occupied property. The quotation should say how the site will be set up, whether skips and waste removal are included, and how access, deliveries and storage will be handled.
Discuss protection to existing floors and finishes, dust control, temporary security and how the contractor will maintain safe access to your home. If a wall is being removed between the extension and your current living space, ask when the property will be opened up and what temporary measures will be used.
Look closely at allowances, exclusions and VAT
A quotation can be detailed and still contain estimated elements. These are often shown as provisional sums or prime cost allowances. They are useful where a final choice has not been made, such as tiles, kitchen units or a particular style of light fitting. The key is knowing the allowance amount and what happens if your chosen item costs more or less.
Ask for exclusions to be written down, not explained only in conversation. Common exclusions can include planning fees, building control charges, kitchen supply, landscaping, specialist surveys, decorating or final connections by utility providers. Knowing this early lets you build a realistic overall budget.
Check whether VAT is included in every figure. Homeowners should also ask whether the quotation covers all labour, materials, plant hire, waste disposal and delivery charges. A price that appears lower at first may simply leave more costs for later.
Understand programme, payment and responsibility
Price matters, but it is not the only decision. Ask when the contractor can start, how long the work is expected to take and which stages are likely to affect access to kitchens, bathrooms or gardens. A realistic programme should allow for lead times on steels, windows, doors and inspections, as well as weather-sensitive external work.
Payment stages should relate to visible progress rather than vague dates. For a larger extension, this may include an agreed deposit, then staged payments after groundworks, structural completion, weatherproofing and final completion. Make sure you receive paperwork for payments and understand what has to be completed before each stage is due.
You should also know who is managing the project day to day. A coordinated team reduces the risk of gaps between building, roofing, electrical, plumbing and heating work. Ask who your main contact will be, how variations are agreed and how often you can expect progress updates.
Compare like for like, not just the final total
Put each quote beside the same checklist and mark what is included, allowed for and excluded. If one price is noticeably lower, do not assume it is better value or that something is necessarily wrong. It may be based on a different specification, a tighter allowance or a different understanding of the plans.
Give each contractor the opportunity to clarify their quotation. A straightforward question such as, “Does this include drainage alterations, final electrics and internal decoration?” can reveal differences that are not obvious from the headline figure. It is better to resolve them before work starts than to debate them after the structure is built.
For homeowners across Coventry, Warwickshire and the West Midlands, IR Building & Construction approaches quotations around a clear site assessment and defined scope. That gives you a practical basis for decisions before trades are booked and materials are ordered.
The right quotation will not promise that every unknown can be eliminated. Older properties can reveal surprises once work begins. What it should do is show how those situations will be handled, who will discuss the options with you and how any change will be priced before it goes ahead. That clarity is what keeps an extension project controlled from the first conversation to the final handover.
