What drives the cost of a home extension on the Northern Beaches

Why two extensions on the same street cost different money

Most people start with a number in their head and work backwards. It is the wrong way round, because on the Northern Beaches the site decides more of the cost than the floor plan does. Two houses a hundred metres apart can come back with quotes a long way from each other, and neither builder is wrong.

Here is what actually moves the number, in roughly the order it tends to matter.

How the land sits

A flat block with a wide side path is the cheapest version of any extension. Everything else is a variation on getting materials in and spoil out.

A sloping block means retaining, deeper footings and often a suspended floor rather than a slab. Around Manly, Fairlight and Balgowlah that is the normal condition rather than the exception, and it is usually the single biggest line item people have not accounted for.

What is already under the house

Sandstone is the one that catches people out. Hitting rock during excavation moves you from a machine to rock saws or hammering, and that is time rather than materials. It is also the reason an honest builder will not give you a firm excavation figure before anyone has dug.

Older homes bring the other version of this: existing footings that were never designed to carry a second storey, or stormwater that has to be rerouted before anything else starts.

Whether a truck can get near it

Access is cost. If a concrete truck can reach the pour, that is one price. If it has to be pumped over the house, or barrowed down a side path, that is another. Narrow streets, no on-street parking and a long carry all add up, and none of it shows on a floor plan.

Which approval path you are on

A complying development certificate is faster and more predictable than a development application, but it only applies if the project fits inside the rules exactly. Step outside them on height, setback or floor space and you are into a DA, which means time, consultants and a council assessment.

Heritage conservation areas change this again. A good part of the Northern Beaches sits inside one, and it affects what you can do to the street-facing part of the house, what materials are acceptable and sometimes whether a second storey is possible at all. Worth knowing before you fall in love with a design.

What you are joining onto

An extension has to meet the existing house, and the existing house was built to the standards of its day. Levels rarely match. Roof lines rarely match. Older brickwork is a different size to what is made now. The join is often the fiddliest part of the whole job and it is invisible in the finished product, which is why it surprises people on a quote.

The fit-out, which is the part you control

Structure is largely decided by the site. Finishes are decided by you, and this is where two otherwise identical projects separate. Tiles, joinery, glazing, tapware and flooring have an enormous range, and the same room can be built to very different budgets without changing a single wall.

It is worth being honest with your builder about the finish level early. A number that assumes standard selections is not comparable to one that assumes imported stone, and comparing two quotes that assumed different things is how people end up thinking a builder was expensive.

What to ask before you compare quotes

• Is the excavation a fixed figure or an allowance, and what happens if there is rock
• What finish level has been assumed, and is there a schedule of what is included
• Are consultant and council costs inside the number or separate
• What is the assumption about access and craneage
• Who is responsible if the existing structure needs work once it is opened up

Two quotes that answer these differently are not really two prices for the same job. Getting them onto the same basis is usually more useful than negotiating either one down.

Where we work

We build across the Northern Beaches and the Eastern Suburbs. If you are planning an extension, our builders in Manly and builders in Queens Park pages go into what we do in each area, and the home extensions page covers the process from first drawing to handover.

If you are at the stage of working out whether a project is feasible at all, that is a conversation worth having before you pay for drawings.

If the house itself is the problem rather than the space, read what to check before a knockdown rebuild on the Northern Beaches.