Your guide - basement conversions







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Basement conversions are becoming increasingly popular in densely populated areas, where land is at a premium and there is no option to extend into the garden or loft space. Basement conversions can be used for anything from a gym to a family room and are a good option to consider if you don't want to move. 

If you want to find out some more, our handy guide is here to offer a helping hand… 

What types of basement conversion can I get?

There are two main types of basement conversion to an existing property:

  • Renovate an existing basement or cellar
  • Create a new basement in your home

Image source Basement Masters

Image source Basement Masters

Who will be involved in my basement conversion?

1. Basement conversion specialists

A very popular option is to engage with a specialist basement conversion company who can manage everything from the design, structural calculations and through to the build. 

A reputable basement conversion specialist will take away the hassle and offer a single point of contact throughout the process. They are also experienced in basement conversions and will be able to provide solutions to any issues that crop up as well as navigate the planning process. Basement conversions can be challenging so it’s worth looking into a specialist company. 

2. Architect or architectural technologist

If you don't want to work with a basement company, a qualified architect (ARB) or architectural technologist (CIAT) can kick off your project. 

An architect (ARB) is the best route if you need design inspiration on your basement conversion, particularly if you need advice on incorporating natural light and design. However, if you have a clear brief then an architectural technologist is a great option. Architects (ARB) typically charge the highest rates, so only use one if you need their design skills, otherwise opt for an architectural technologist (CIAT).

Either way, make sure you choose one who has experience working on basement conversions.

3. Structural Engineer

A structural engineer will be responsible for producing the structural drawings and calculations that are required for building regulations. Structural engineers are always involved in your basement conversion as you must adhere to building regulations. Your basement conversion company or architect will normally have one that they work with.

4. Surveyor

If your basement conversion is in a run down property or a period property then it’s worthwhile appointing a surveyor at the outset to identify any issues that you may otherwise overlook. A surveyor will also be able to help with other issues including the Party Wall Act (England and Wales only). 

You can also get a geology survey to determine how suitable your property is for a basement conversion. 

5. Builder

When hiring builders make sure they have experience working on basement conversions - your basement conversion specialist or architect should be able to recommend some. You can either choose a builder who can do everything and employs specialists (such as plumbers and electricians) or you can employ individuals and project manage the process. 

Be aware that project managing various tradesmen that haven’t previously worked together might be challenging.

Image source Basement Masters

What do I need to consider before planning a basement conversion?

There are many issues that can impact the feasibility and cost of a basement conversion which need to be considered before you begin, particularly if you don't have an existing basement or cellar. Seek advice from a specialist basement conversion company or architect for advice on your property. Some of the issues you should discuss are as follows:

1. How will you access your basement?

Consider how you will access your basement from the original ground floor and also your fire escape methods. Also consider if you would like access onto your garden - this is a great way to introduce natural light.

2. What is the minimum ceiling height you need?

2.4 metres is recommended in a basement if you want to create a comfortable living area.

3. How will you connect to drainage?

You will need to consider the impact of your conversion on the existing drainage and whether you will need to divert them as part of your conversion.

4. Conditions of your plot

It's important that you consult an expert to assess your plot. Factors such as the local water table will impact the type of basement structure and the kind of waterproofing used to protect your basement against flooding or dampness.

A site survey will be able to determine your local geology and determine the best structure for your basement, identifying any issues such whether it sits on clay or marshy land. 

5. How will you maximise light into your basement?

Unless your basement is going to be used for a cinema or wine cellar, you will want to maximise the light to make it into a light living space. 

To maximise the light, you would create a sunken garden space where your basement leads out to although this will add significantly to the budget. Multiple lightwells are a very popular way to introduce light too.

What kind of home is a basement conversion most suitable?

If you have an existing cellar or basement then a conversion will be fairly straightforward. However, you can create a new basement to an existing property and this is very popular in densely populated areas such as London.

Do I need planning permission?

If you are altering an existing cellar or basement then you might not need planning permission, particularly if you are not changing the appearance from the outside. However, if you are creating a new basement then you will most likely need planning permission. 

If you’re considering converting your basement, you should always consult your local authority and confirm whether your plans are within permitted development.

To find out more about planning permission, read our article your guide - planning permission.

Do I need building regulations

Regardless of whether you need to apply for planning permission, you will need to comply with building regulations. Building regulations do not supervise the builders work but ensure it meets a minimum standard.

Building regulations checks will include:

  • The new structure
  • The fire resistance
  • The access and escape means 
  • The insulation and ventilation
  • Damp proofing
  • Water supplies

A structural engineer will be able to complete the drawings required for building control and often can apply for building regulations on your behalf, but do confirm that they are doing this. You should get a completion certificate 8 weeks after completion - make sure you receive this and file it.

How much does a basement conversion cost?

If you are converting an existing cellar or basement, expect to pay £1,000 - £1,500 per square metre for the labour and build (the cost will vary depending on the condition of the existing space). 

If you are creating a new basement to an existing home, expect to pay between £2,500 - £3,000 per square metre for the labour and build.  

On top of these costs you will need to budget for the following:

  • Professional fees of between 10% - 15% of the labour cost. If your property is impacts the Party Wall Act then you should budget additional costs to appoint a surveyor.
  • VAT costs of 20%
  • Fitting out the conversion including a bathroom, flooring, lighting, plastering and decoration. This depends very much on the look you want and can range substantially.

Basement conversions are often the most expensive conversions when you are creating a new basement. If you live in a densely populated area, such as London, then it's often a worthwhile investment as land is scare and property value is at a premium. However, in many cases a loft conversion or extension is a better investment. We recommend asking a local estate agent to visit your home and advise you on whether a basement conversion will add value to your home before committing to the work.

How long does a basement conversion take?

Converting an existing cellar or basement may take as little as 1 month if it's in good condition. 

Creating a new basement will take 6 -9 months from the initial planning through to the actual construction. Expect the actual construction to last up to 6 months, depending on the complexity.

Anything else I need to consider?

You need to be aware of the Party Wall etc. Act 1996 (England and Wales only) which often applies to basement conversions. Your architect will be able to advise you if this impacts you and should be able to recommend a surveyor, should your neighbour dissent the notice. If you need to know more then read our article expert advice - party wall act. In Scotland and Ireland, boundary rules apply.

Published: February 4, 2019

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