Window extensions

Basement Conversion Guide

The Sterlingbuild Guide to Basement Conversions

Do not place a limit on what you can use your basement or cellar for. A basement conversion or brand new basement extension can provide you with space for a variety of different rooms. Here is how.


Why you should extend your basement

If your current living space is sparse and you are looking for somewhere to build that gym room or office space you have forever been longing for, the solution you are seeking may be right under your nose, well feet anyway.

The majority of basements can be excavated or converted and can provide you with often the most attractive and largest living area in your home, adding value to your property at the same time.

If you already own a cellar, renovating it could be a sensible move to gain room while not having to sacrifice any exterior or garden space. In most cases, a basement conversion will not require basement planning permission, but be sure to double-check this with your local planning authority.

If you’re more the India Jones type and are planning on a basement excavation to create a new basement, planning permission will likely be needed.

basement extension ideas with Lightway sun tunnel
new lodge idea for basement extension using walk on rooflights

Basement ideas

Walk-On rooflights, able to be installed internally and externally, are very popular with basement conversions as they enable daylight to reach areas of the building which might otherwise be dark by introducing light from above. We have a range of walk-on rooflights available in a variety of designs and sizes from Signature and FAKRO.

If your cellar conversion or extension is part of an open-plan design running through the building, Korniche roof lanterns could be the perfect choice for you due to their ability to optimise the amount of light that enters from outside –  creating a much stronger source of illumination and lessening the needs for bulbs.

If situated on a slope, you could include REAL aluminium bifold doors or sliding doors  opening out onto a courtyard.   

Basement lighting 

With basements synonymous as being dark and unwelcoming places, whatever type of room you are designing with your basement remodel, implementing the right lighting is essential to creating a bright and welcoming environment. 

Letting natural light into your basement is always the preferred option as this goes a long way to not only making the space inviting, but healthier too.

As explained above in our ideas section, a great way to let daylight into your new room is with walk-on rooflights, possibly the best, and sometimes only, choice for basement extensions and conversions.

The Lightway F400 through wall sun tunnel is an extremely practical choice for cellars and will provide more than enough natural light to negate the need for switching on lightbulbs during the day. All Lightway sun tunnels are fitted with high-quality Bohemian crystal for an extra touch of class.

Sunsquare walk-on rooflights on patio exterior of home
Walk-on rooflights for basement conversions located inside the home

How much does a basement conversion cost?

If you are converting an existing basement space and do not need to do anything structural, the cost of renovating a basement can total around £1,000 to £1,500 per square metre, although naturally this will go up should you need to raise or lower floors. 

A basement excavation would be more expensive due to the extra work needed. As well as building costs, other outgoings that need factoring in include, basement waterproofing, underpinning walls and rerouting sewers or drains.

The cost of building a basement can range between £2,500-£4,000 per square metre. Prices will vary depending on area. A London basement, for example, will cost more to build.

Important things to consider

Creating a safe working environment for building is a necessity when expanding or converting a basement, especially if excavation work is being carried out. It is a high-risk job that if not carried out safely can lead to injury and the stability of the existing building being ruined.   

An example plan can be viewed online on the Health and Safety Executive along with guidance on best practice.

Fire safety also needs considering. This can be managed by an external door or window suitable for exit from the basement or a protected stairway leading from the basement to a final exit.

Contact a ‘building control body’ (BCB) for a full list of building regulations appropriate to your property. This can be done by visiting

For help finding basement builders and basement construction firms, visit the British Association Sites.

Lightway through wall sun tunnel great for filling basements with natural light