• Draw Plans


Updated: Aug 28, 2020

Congratulations for finding our guide for basement conversions. There is quite a bit of information online already about converting your basement but most of the articles I read offered up more questions than answers and I couldn’t find one article that gave you that walk through feel that left you with a well-rounded grasp of whether a basement conversion was in fact a money pit or a decent investment.

I hope our 25-point guide to basement conversion helps to provide you with a better understanding of the process and the merits of investing in a basement conversion.


Location is the number one factor that determines if a basement conversion makes financial sense. For example, if we were to carry out a basement conversion in Clapham London SW4 the value added would be around £6,025 per m2.

If we did the same basement conversion in Sydenham London SE26 the value added would be around £4,885 per m2.

If we went down the country a bit to Weybridge in Surrey that same basement conversion would add around £5,922 per m2.

So, by now you should be getting the hang of it and with a little bit of research you can look at any property anywhere in the UK and calculate the value added per m2 in minutes.

But knowing the value added per m2 is not enough as you also need to know the cost per m2 of doing a basement conversion and for that I can tell you that the current price range is around £4.5k to £6.75k. (July 2020)

Now if you get your calculator out you will be able to work out that a 50 m2 basement conversion in Clapham London SW4 will add around £300k in value.

The same conversion in Sydenham will add around £245k and in Weybridge about £295k.

When it comes to basement conversion costs, I have given you a range of £4.5k to £6.75k per m2 simply because each site is different in complexity and the amount of work to be done to achieve the goal.

Based on the cost figures provided you are looking at somewhere between £225k to £335k to get the job done.

So, if the conversion was straightforward then you might make an instant profit but on the other hand if it was more complex you could be out of pocket by the time the conversion is complete.

But the analysis here is not based on instant profit, it is based on whether a basement conversion is a good investment and to answer that we have to look at property prices and make a determination as to whether property will rise in value over the coming years and if so, by how much.

To get a clearer picture let’s take a look at how property prices have risen in the UK over the past ten years 2010 / 2010.

So, on the whole, property prices have risen by 33% in the UK over the past decade but if we focus on London, we can see that property prices have doubled the national average and hit 66%.

To get an even clearer picture of the property market let’s see which parts of London and the UK have had the highest property growth since the millennium 2000 / 2020.

Source Halifax
Source Halifax

So by looking back at property price rises over the past 20 years we can see that the London Borough of Newham has topped the chart with a 429% price change. This is closely followed by London Borough of Waltham Forest at 418% and London Borough of Lewisham at 379%.

What all this means is that had we invested in a basement conversion in the London Borough of Newham back in 2000 we would be looking at a Return of Investment (ROI) of 429%.

Had we done the same basement conversion in Brighton & Hove where it is likely the cost of the basement conversion would have been lower, we would have seen a Return of Investment (ROI) of 315%.

Compare that to Gold which had a price rise of 81% over the past decade or Oil which went down by over 100% and you start to understand why people continue to invest in property.

Of course all those figures relate to what would have happened had we carried out a basement conversion during the past 20 years. But what about now?

The answer lies in who you are going to believe with regard to property rise forecasts for the next decade. Here we have a forecast based on Savills and This is Money.

If you research the internet you find varying figures that predict UK property growth over the next five years. Clearly corvid-19 will have a big effect over the next year or so but in the long term we have to look to the main causes of property rising in value such as low interest rates and housing shortages to draw our own conclusions as to where the property values will go in the future.

Do keep in mind that whilst we have National price forecasts of around 50% over the next 5 it is the regional prices you need to be focused on because based on the historical data London will be the place where the real profits lie.

Once thing is certain, if you wish to invest in a basement conversion make sure you select the right location as the location is the key to getting the highest return on your investment.


Would you believe me if I said that up until a few years ago most basement conversions were permitted development? Yes, it is true. Nowadays most basement conversions require a planning application particularly if you are excavating deeper or wider than an existing cellar or basement.

What makes matters a bit tricky is that some of the rules can vary across different planning authorities which means you should really have a word with the local planning officer about your basement ideas before instructing your architect to create basement conversion plans.


A geo tech survey is essential to determine soil conditions where you will be digging out.


Another important survey to reveal any drains, pipes and sewers below your property.

Your project may need approval from the local water authority so now is a good time to make the application.


The topographical survey is all about measurements. This is required for accurate measurements and mapping out of the site to produce accurate plans for your basement conversion.


Another requirement to determine plant life underneath the property. It is also used to determine if nearby trees will be affected by the development.


A basement impact assessment is now a requirement by most councils. This is to provide a wider picture to the council during the planning approval processs with regard to hydrogeological conditions, risk of surface water flooding and slope stability. These are just some of the concerns that will be looked at for the assessment.


Not always required. Your local planning authority will advice on whether you need one done.


Now it starts to get interesting. This is where you meet with designers to explore your ideas and to create a design concept for the project.


Basic layout and design plans are created for the project.