There's no such thing as a fixed contract in building.

Every building project begins with a number. Clients understandably want to know what the project will cost, and main contractors work hard to prepare estimates that reflect the drawings and specifications in front of them. Then there is the real world, that only tends to appear when the hammers are swinging and the materials are flying.

Construction is one of the few industries where the final product is being created on a site that cannot be fully understood until work begins. The ground hasn’t been opened up yet, hidden parts of existing buildings haven’t been exposed, and multiple trades haven’t started interacting with one another in real time.

Because of that, building costs are not just about the design or the materials. They are also about uncertainty.

This blog is the first in a short series looking at the common areas in a project where costs tend to move during construction. The goal isn’t to scare anyone off building. Quite the opposite. When you understand why variations occur, they become far less stressful and far more manageable.

Start your build budget with a contingency - 15% is usually about right.

Contingency is often misunderstood as a vague buffer or a sign that something in the design isn’t certain. In reality it is simply a practical allowance for the small adjustments that naturally occur when turning drawings into something built on a real site.

Every experienced builder expects some level of movement during a project, and contingency is the mechanism that allows those changes to be absorbed without derailing the budget.

For that reason, it’s best factored in early, when the overall budget is first being considered, rather than treated as an afterthought.

Here are a few of the most common areas contingency comes into play:

Right at the beginning

Before foundations can be built, the site needs to be prepared. Earthworks contractors shape the platform, drainlayers install services, and foundation contractors require a level, compacted surface to work from. Each trade works with different tolerances, and the ground often reveals surprises once excavation begins, groundwater, softer soils, buried material or services.

Earthmoving equipment is designed for moving soil efficiently, but not finishing with millimeter precision. Drainage trenches can expose conditions that require deeper digging or additional work.

By the time the foundation contractor arrives, the platform often needs trimming, filling or further compaction to achieve the accuracy required for structural work.

Nothing has necessarily gone wrong when this happens. It’s simply the reality of several trades working sequentially in an environment that can’t be perfectly predicted until it’s opened up.

This is why site preparation is one of the areas where estimates most commonly move during a build or are omitted from fixed price contracts.

Onsite variations

Onsite variations are changes that occur after tendering or during construction as the building takes shape. Typically they are practical decisions made between a client, builder, subcontractors and the designer to ensure the construction proceeds smoothly and the outcome at the end is as anticipated.

A minor example is a framing member shifting slightly to accommodate services. A slightly larger variation may be a window converting into a door. In some cases, a flashing detail might be adjusted to suit the way materials have been installed or often a junction between two materials is refined so the finish looks cleaner or performs better.

Our construction drawings aim to anticipate as much as possible, but no set of documents can describe every screw, every cut or every sequence of work that will occur on site. Builders and trades bring a different perspective to the project, suggesting small improvements or adjustments as the project progresses.

In renovation projects this happens even more often. Once parts of the existing building are opened up, the structure behind linings can differ slightly from what was expected. Timber sizes, framing positions or earlier alterations may require small changes to the way new work connects to the old.

Most onsite variations are modest in scale. They may involve a small amount of extra labour, a slight material adjustment, or a minor design tweak to improve buildability.

On occasion variations require council amendments and additional drawings, these tend to be a little more expensive to execute.

Installation of services

Once the structure of a building begins to take shape, a large amount of work starts happening behind the walls, ceilings and floors. Plumbing, electrical, data, heating systems and drainage all need to be installed and coordinated within the structure.

Part of the challenge is coordination. Services often run through framing, floor structures and ceiling spaces that are shared by multiple trades. A pipe or duct may need to shift slightly to avoid a beam, a switchboard location may need adjustment once framing is complete, or additional wiring may be required once the electrical layout is finalised on site.

In renovation projects the uncertainty increases further. Existing buildings rarely match the drawings perfectly, and hidden services can appear once linings are removed. It is not uncommon to discover older plumbing, wiring or drainage that requires upgrading or rerouting to meet current standards.

There are also situations where the exact requirements only become clear during construction. Clients often decide to add additional lighting, power points, data outlets or features once they can physically walk through the framing of the building. These are often sensible improvements, but they do alter the scope of the services installation.

Because much of this work is concealed within the building, changes can require additional labour or materials before walls and ceilings are closed in.

Like the earthworks none of this means the project has gone off track. It reflects the reality that modern buildings rely on a complex network of services working together within a relatively tight structural space.

Fixtures and fittings

Fixtures and fittings cover the parts of the building people interact with every day: kitchens, tapware, appliances, lighting, door hardware, bathroom fittings, wardrobes and cabinetry.

Early in a project these items are often allowed for as PC Sums or allowances. At the time the builder is pricing the job, not every tap, light fitting or appliance has necessarily been selected. Instead, a reasonable budget is included based on typical products.

The variation occurs when the real selections are made.

Once clients begin choosing specific items, the price can move in either direction. A tap that looked similar in a showroom might cost twice as much as the allowance. Appliances may change once the kitchen layout is fully understood.

None of this is unusual. Fixtures and fittings sit at the intersection of function, aesthetics and personal preference, so they tend to evolve as people begin to experience the spaces being created.

There is also a practical aspect to installation. Some products require additional supports, different plumbing connections, or more electrical work than initially assumed. A wall-hung vanity, for example, needs different structural backing than a floor-mounted unit. Feature lighting may require additional switching or wiring.

Because these items sit toward the later stages of construction, they are one of the most visible areas where the project cost can shift slightly.

Finishes

Finishes include the visible surfaces of a building: flooring, wall finishes, paint systems, tiling, carpets, timber finishes and exterior coatings. These materials define the look and feel of the space, and they are often chosen later in the design process once the overall character of the project becomes clearer.

Much like fixtures and fittings, builders often allow a reasonable estimate for finishes during pricing. At that stage the exact tile, flooring system or paint finish may not yet be confirmed. Once selections are made, the final cost can move depending on the products chosen.

Tiling is a good example. Tiles themselves can vary significantly in price, but so can the labour required to install them. Larger format tiles, feature layouts or complex patterns often require more time and precision than a straightforward installation.

Flooring can introduce similar adjustments. A simple carpet installation is very different from laying engineered timber or polished concrete. Subfloor preparation can also play a role, particularly if small variations in the surface need correcting before the finish material is installed.

Wall finishes sometimes reveal the reality of construction tolerances as well. Plastering and painting can require additional preparation if surfaces need extra attention to achieve the level of finish expected in a completed home.

None of this suggests a problem with the project. It simply reflects that finishes sit at the very end of the building process, where the craftsmanship and materials that define the final appearance come together.

Landscaping

Landscaping is often one of the final stages of a project. It includes driveways, paths, lawns, planting, retaining walls, decks, fencing and outdoor living areas. These elements shape how the building sits within the site and how people actually move through and use the outdoor spaces.

Early in a project, landscaping is frequently allowed for in broad terms rather than fully detailed designs. The focus during consent and construction is typically on the building itself. As the project progresses and the site becomes clearer, the landscaping plan often evolves.

Levels may need adjusting where excavation has occurred. Drainage solutions sometimes need extending to manage stormwater properly. Retaining walls might become necessary once the final ground levels are established. Even something as simple as a driveway can vary depending on materials and site preparation required.

There’s also a natural tendency for outdoor spaces to develop as people start imagining how they will use them. A basic patio might grow into a larger entertaining area. Planting plans may expand once the scale of the garden becomes apparent. Fences or screening can be added to improve privacy after the building is complete.

Landscaping also sits at the mercy of ground conditions and weather. Soil quality, drainage and access can all influence how straightforward or complex the final works become.

So for a successful project, allocate your contingency first.

Construction is not an assembly line; it’s a collaboration between drawings, materials and the people building them. Onsite variations are simply the small adjustments where experience and practicality refine the building as the design becomes real.