How to Avoid Cost Overruns on Building Projects

Build a Proper Preliminary Budget

What Most People Leave Out

A realistic project budget needs to cover more than bricks and labour. The items that get missed most often are not obscure ones. They're obvious costs that people choose not to include because they don't want the total to go up.

Professional fees: Architect or designer (typically 8–15% of construction cost for a full service), structural engineer (£1,500–5,000 for residential extensions and new builds), planning consultant if needed (£1,000–3,000), building regulations application (£250–1,500 depending on scale), party wall surveyor if applicable (£700–2,000 per neighbour).

Enabling works: Site clearance, demolition, asbestos survey and removal if required. Asbestos is present in a very large proportion of buildings constructed before 2000. Always commission a Type 2 survey before starting work on an existing building. Ground investigation costs £800–3,000 for a residential site but can prevent an extremely expensive surprise when the excavator hits something unexpected at foundation depth.

Utilities: Connecting to mains water, gas, and electricity is not included in any contractor's price unless specifically stated. A new mains water connection costs approximately £1,500–4,000. A new electricity connection: £2,000–8,000 depending on distance from the network. Gas main connection: £2,000–6,000. These are utility company charges, not contractor charges, and they vary enormously by location and network capacity.

Scaffold: A correctly priced building project includes scaffold in the preliminaries. Many self-builders get a separate quote late and are shocked by the figure. A three-storey house in scaffold with lift throughout: £4,000–9,000. An extension with roof-level access: £1,500–4,000. Scaffold hire runs weekly. If the programme slips, the scaffold cost keeps running whether anything gets built or not.

Plant and equipment: Excavator hire, telehandler (Manitou), concrete pump, generator. These are either in the contractor's preliminaries or charged separately. If they're not in the quote, ask where they are.

Skip hire and waste disposal: Easily £2,000–8,000 on a medium-sized project across the full works. See our skip size calculator for estimates.

VAT: New build residential construction is zero-rated for VAT. Extensions and conversions are standard-rated at 20%. A self-builder can reclaim VAT on new-build materials through the HMRC DIY Housebuilder Scheme, but only if the project qualifies and the claim is submitted correctly within three months of the building being complete.

Contingency: How Much Is Enough?

The industry standard contingency for a well-designed, well-specified residential project with a competent contractor is 10% of the construction cost. Use 15–20% if you're working on an existing building (especially anything pre-1980), have uncertain ground conditions, a tight programme, or a client who is likely to make changes during construction. Simple new build on a clean site with a fixed-price contract: 10% is workable. Victorian terrace renovation where you genuinely don't know what's behind the walls: 20% is sensible.

Contingency is not a slush fund for things you want to add. It exists for genuine unforeseen conditions: unexpected foundations, hidden drainage, structural issues, material price spikes. When contingency gets drawn down by scope changes, the budget needs to go up. Drawing down the contingency and hoping nothing else goes wrong is how projects finish in dispute.

Managing Variations: The Hidden Cost Killer

What a Variation Actually Costs

A variation is not just the cost of the changed element. It carries overhead costs that rarely make it into the variation price at first glance. Ask a contractor to move a stud wall after it's been framed out. Direct cost: maybe £200 in materials and four hours of labour. Indirect costs: repricing and redrawing time, the plumber who was booked for the following day who now can't come, materials already purchased that go to waste, the site manager's time processing the change. It adds up.

Competent contractors price variations with a 20–30% markup above direct cost, which is commercially fair given the disruption involved. Clients who get a variation price and complain it's expensive should understand one thing: the best way to avoid expensive variations is to make decisions at design stage.

How to Minimise Variations

Fix the design before you go to tender. Every drawing that says "to be confirmed" or "client to select" is a future variation. Specify kitchens, bathrooms, floor finishes, window types, and ironmongery before the contract is signed. Decisions made during construction cost more than decisions made during design. Sometimes dramatically more, if the contractor has already ordered materials or done preparatory work.

Walk the site with the contractor and the drawings before work starts. A pre-start meeting to check what the drawings show against what actually exists on site takes an hour. Finding the same discrepancy mid-construction takes weeks to resolve. That's not an exaggeration.

Agree a variation procedure in writing. The JCT Minor Works contract includes a straightforward mechanism: the contractor must give a price before carrying out a variation, and the client must approve it in writing. Use this. Never allow work to proceed on "we'll sort out the cost later." That's how disputes start, and how they end up in adjudication.

Programme Delays Cost Money

Programme delay is the cost overrun most people forget to account for. A main contractor's preliminaries, site management, plant hire, scaffold hire, welfare facilities, insurance, keep running every week the project is on site whether or not productive work is happening. On a medium-sized project with a resident site manager, prelims run at £3,000–6,000 per week. Six weeks of overrun adds £18,000–36,000 before a single additional brick gets laid.

The causes within the client's control: late decision-making, approval delays, and design changes. Within the contractor's: poor subcontractor management, late ordering, and inadequate site management. Both parties have a strong financial interest in running to programme. That shared interest is worth reminding both sides of regularly.

Choosing the Right Contract Type

Fixed Price (Lump Sum)

A fixed-price lump-sum contract gives the client certainty on construction cost. The contractor carries the risk of any increase in materials and labour within the agreed scope. This is the standard approach for most residential extensions and new builds where the design is complete before the contract is signed.

Fixed-price contracts only work properly when the scope is genuinely fixed. A specification full of "to be confirmed" and "provisional sum" items is not a fixed price. It's a fixed-price framework with a variable scope, which produces a variable final account. The more complete the specification, the more meaningful the fixed price becomes.

The main risk for clients on fixed-price contracts is selecting on price alone. A contractor who comes in significantly lower than their competitors has either found genuine efficiencies or priced the job incorrectly. Both outcomes need investigating. A contractor who cannot deliver at their quoted price will either use variations to recover their margin (legitimate if changes are real, problematic if the contract is being used to fix a mispriced tender) or will fail mid-project. A contractor failing mid-project is the worst outcome possible.

Cost-Plus (Prime Cost)

Cost-plus means the client pays actual labour and material costs, plus a management fee of typically 10–20%. This approach makes sense where the scope is genuinely uncertain, particularly on renovation and conversion projects where hidden conditions are likely to change everything. It's also appropriate where the client wants transparency and is prepared to accept cost uncertainty in exchange for it.

The risk is straightforward: there's no financial incentive on the contractor's side to manage costs tightly if the fee is a percentage of whatever gets spent. A competent client or project manager can deal with this by reviewing invoices, agreeing labour rates in advance, and requiring competitive quotes for subcontract packages. A target cost mechanism, where the contractor shares in any saving against a budget, aligns incentives better than a pure percentage arrangement.

JCT Minor Works

For most residential work in the UK, the JCT Minor Works Building Contract is the right form to use. It covers scope, variations, payment, insurance responsibilities, and dispute resolution in a clear and widely understood framework. It is not complicated. Contractors, architects, and solicitors all know it. A bespoke letter-of-instruction arrangement might feel simpler at the start but it creates ambiguity that becomes expensive when something goes wrong. The cost of completing the JCT correctly is a few hours of professional time. The cost of a payment dispute on a £150,000 project without it can easily exceed that many times over.

Procurement Timing: Buy Early, Buy Right

What to Order Early

The materials with the longest lead times, bricks, roof trusses, windows and external doors, structural steel, should be ordered the moment the design is fixed and planning permission is in hand. Waiting creates two problems at once: programme delay because the work cannot proceed until the materials arrive, and price risk because material prices can move significantly over a 3–6 month period.

In practice, order bricks and trusses at or shortly after contract signing. Order windows and external doors as soon as U-values and opening sizes are confirmed, not when the walls reach window-head level. A specific aluminium or timber window system in a particular colour or profile can have 6–10 week lead times. Ordering late is one of the most common and most avoidable causes of programme delay on residential projects, and the one clients are most likely to underestimate.

Buying Ahead of Programme

For larger projects and developers running multiple schemes, buying materials ahead of programme and holding them in dry, secure storage can produce significant savings. Plasterboard, insulation, and timber can all be purchased at volume prices and held. The storage cost runs typically £150–350 per week for a small warehouse bay. That is almost always less than the premium for emergency ordering or repricing mid-project.

Forward purchasing only works if your take-off quantities are accurate. Use the calculators on this site to confirm quantities before ordering. Under-ordering means emergency top-up deliveries at premium prices. Over-ordering means surplus material that has to be credited or disposed of. Getting the quantities right before you pick up the phone is where the real saving is made.