Reader Clinic with Jeffery & Wilkes

Reader Clinic with Jeffery & Wilkes

Answering all of your questions on self-builds, conversions, extensions, and everything in-between! Get in touch with your queries by emailing us at

In this post we focus on extensions with expert advice from Greg Wilkes, the managing director at Jeffery & Wilkes, a design and build company that specialise in loft conversions, home extensions, and home refurbishments.

What size extension can I build without planning permission? Donna, via email

Under Permitted Development (PD) rules you can extend the full width of the rear elevation to a depth of 3m for an attached property and 4m for a detached property. There are other limits and conditions that can be found on

Do I need an architect to plan an extension? Tom, via Twitter

It’s highly advisable to get drawings carried out to satisfy both the planning department and building control. This gives you the peace of mind that if your builders follow these plans, the extension is being built to the right size and in the right way to comply. It will also be easier to sell the property with the plans in place. However in theory you do not require drawings by an architect and if your builders are extremely competent, it could be built in accordance with regulations without any drawings or specification. Some companies will offer a Design & Build service which will mean you do not have to engage a separate architect and this could be an avenue worth pursuing to have everything under one roof.

How much does it cost to build an extension? Rebecca, via email

This will vary around the UK, but in London expect to pay between £1200-£1800 per sqm. There are a multitude of factors that will add additional costs like finishes, roof style, and glazing.

How long does it take to build an extension? Derek, via email

An average rear single storey extension of 5x4m should take around 8 weeks from foundations to decorating. A double storey would be between 12-16 weeks depending on scale and specification. Bear in mind that weather related issues could increase the timings; brickwork cannot be laid in freezing temperatures, and if it were a particularly wet and cold season, drying out times would be longer. This could impact timings on things like plaster and screed as they need to fully dry out before finishes are carried out. There could also be unforeseen problems like underground drainage requiring alteration that would delay time on site. However surveys can be undertaken at additional cost to limit these types of issues. 

Can I extend on an existing extension? Chris, via email

Yes but the above limits would still apply under Permitted Development (PD). So if you have already extended you may only have a small amount of PD allowance remaining.

You can apply for Planning Permission to extend further.

How much value can an extension add to my house? Holly, via email

A well thought out extension will always add value and this can be as much as 20% increase on the property value, but price caps on the area will apply. You can check the sqft value of your home on websites like Zoopla which will give you a guide to calculate the increase in value of your extension.

To discuss your project or property please call Jeffery & Wilkes on 020 8819 3883 or go to their website