ARTICLE Stephanie Matheson

Excavation renovations and basement conversions could cost approximately $250,000* to $500,000+*. 

With section sizes and lawn space at a premium, plus height and boundary limitations in place in most urban settings, basement conversions are fast becoming a renovation style of choice for creating additional living space.

Where do I start?

Getting architectural plans drawn up for your project is one of the first steps to get your renovation underway. When excavating or converting a basement space, typically engineering work is necessary to provide sufficient support. This is a major cost influencer, so a large percentage of your renovation costs will be determined by the kind of structural and foundation work that will need to be done.

Extending your home below ground level is generally more expensive than a same level home extension because more work is required. You should plan on spending from around $250,000* to $500,000* for a space that’s ready for you to move in to, including materials, labour and project management. Basement conversions can vary greatly in terms of cost factors, so estimates need to be provided on a case-by-case basis.

How do I support and protect my existing house?

The main concern with a basement conversion is how to support and protect your existing house. First of all the existing structure requires propping. In fact, you could spend between $7,000* and $10,000* on support work just to secure your present home, before any excavation work can even start, When the building phase begins, strong foundations need to be put in place to ensure your extension will support the main house. This typically requires steel and concrete structures. The soil type of your property may have an impact on the cost as loose or unstable soil such as ash or sand may require more work.

Progress photo of excavation showing foundations, propping and water tightness

Apart from structural work, one of the most important issues is creating a watertight space. Solid sheet membrane is the material most commonly specified, along with insulation and a system of drainage pipes that lead the water away from your house. It’s worth planning this part of the renovation carefully to ensure your new living space is sealed properly and stays dry long-term.

What tools can I use to reduce labour cost?

While hiring extra machinery incurs additional costs, the use of small excavators can really speed up the renovation process and hence save time and reduce labour costs. A small digger with operator could cost around $300* to $600* a day, and it’s usually necessary to hire a small dump truck along with it, which can add up to an extra $200* to $300* a day. If you get a fixed price quote from your builder, these costs should be factored in. However, there can be unforeseen soil-related problems such as striking rocks or large tree roots that can slow down work, and increase costs.

Earth work and disposal on excavation renovation project

Rubbish removal costs are often forgotten about in the planning stages, though for a large-scale excavation project they could add up quickly and blow your budget (having a contingency budget is key). Access to the building site has a big impact on the cost. For example, if site access is limited it may take two or three times as long to get the excavated soil out.

How can I keep my basement warm?

Ventilation is key for a healthy and comfortable basement extension, so make sure you factor a heating and ventilation system into your renovation plans. This could be as easy as adding a single unit electric heat pump, but can also include underfloor heating which especially practical in a basement as it keep the floor warm and the rooms dry. Be sure to install proper insulation first.

installation of underfloor heating solution

If you have a roof cavity, you could consider installing a ventilation system that would distribute warmer air from the roof to your cooler ground floor and basement rooms. Costs for a reclaim heat exchange system with two intakes and two outlets start from approximately $5,000*.

What lighting should I choose?

Basement rooms can lack sufficient natural light, so it’s important to consider and plan your lighting requirements. Adding large windows and sliding doors to your new space where possible is the obvious and most cost-effective way to fill your extension with natural light. If you are converting space that is directly beneath the house and surrounded by soil on all sides, you may be able to work with skylights or ‘solar tubes’, which bounce light down through tubes and diffuse it at the bottom, to brighten things up. It also helps to choose light colours for your walls, floor and ceiling, and to ensure that your ceiling is similar in height to your upper floor.

large windows and sliding doors fitted on basement conversion to allow in lots of natural light

When it comes to artificial lighting, LEDs are becoming more and more popular as the main light source, and they have a couple of certain advantages for basements. One option is to install LED strips behind a specially created pelmet or false wall. The strips provide a great spread of light and illuminate the whole ceiling. With some lighting systems you can even choose different lighting situations such as sunrise or sunset to suit your mood. The initial cost of LEDs may be higher than for other fittings, but they have a very long lifespan and are more cost-efficient in the long run.

What type of flooring options do I have?

Choosing the right flooring will ensure your extension becomes and remains a comfortable and healthy space. In most cases you will start with a concrete base and can then choose from a wide range of flooring. Depending on your needs and preferences, you may even decide to leave the concrete floor and polish it to your preferred finish. Options include adding aggregates such as stones or shells, and if you budget allows for underfloor heating, this solution is warm and will give your new space a sleek and stylish look and feel. You can also run hot water pipes through the concrete slab, which has a similar heating effect.

Many basement extensions house bedrooms or media rooms where a touch of cosiness is welcome, so carpet may be the best flooring choice. Synthetic carpet is a budget-friendly option, while woollen products offer a natural comfort. In either case, consider opting for a short-pile carpet that will stand the test of time. Electric underfloor heating pads can be installed underneath the carpet too. If you prefer a timber look or want to visually connect the extension to existing wooden flooring, you have a choice between hardwood floors and timber laminates. The latter are typically more affordable.

You might also be interested to read our cost estimates on same level house extensions, or loft conversions and second storey additions.
 

Renovate Magazine LogoThis home renovation advice article featured on page 86 of Issue 016 of New Zealand Renovate Magazine. New Zealand's first and only magazine solely dedicated to home renovations.
 

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*Costs are rough estimates and are subject to change. For a fixed-quote accurate to your specific project, please consult your local Refresh Renovations specialist. All Refresh Renovations franchises are independently owned and operated.

All Refresh Renovations franchises are independently owned and operated.

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