Choosing between a single storey vs double storey extension is one of the biggest calls you will make. You need more space, but you are torn between building out or building up. Get it wrong and you risk wasted money, planning delays and months of upheaval.
This guide gives you straight answers, with no jargon. You will learn the real costs, the Kent planning rules, and how each option affects your home’s value. By the end, you will know which extension suits your home.
What Is the Difference Between a Single and Double Storey Extension?
A single storey extension is a ground floor addition, usually built to the rear or side of your home. It adds space on one level without raising the height of your house. People use it for a bigger kitchen, a dining room, a utility or a home office.
Double storey extensions add new space on the ground floor and first floor at once. You extend both the footprint and the height of your home. This gives you living space below and bedrooms or a bathroom above.
One rule catches people out. A side extension of more than one storey almost always needs full planning permission.
Single vs Double Storey Extension: The Quick Comparison
Here is the short version. A single storey extension costs less and builds faster. A double storey adds more space and better value per square metre. The table below compares both at a glance.
| Factor | Single storey | Double storey |
| Cost per square metre | £1,800 to £3,000 | Around £1,900 (standard) |
| Typical total cost | £30,000 to £70,000+ | Higher total, lower per m² |
| Planning permission | Often permitted development | Almost always required |
| Build time | Around 12 to 16 weeks | Longer than single storey |
| Space gained | One floor | Two floors |
| Garden impact | Larger footprint loss | Less footprint per room |
| Value added | Often 10 to 15% | Usually more (adds a bedroom) |
Sources for these figures include the HomeOwners Alliance and Checkatrade. Costs in Kent and the South East often sit at the higher end.
How Much Does Each Extension Cost in 2026?
As of 2026, a finished single storey extension costs £1,800 to £3,000 per square metre. According to the HomeOwners Alliance, that range is based on Checkatrade data. Most single storey projects come to £30,000 to £70,000 or more.
Several things move that price: size, finish, glazing and structural work.
Is a Double Storey Extension Cheaper Per Square Metre?
Yes, usually. A double storey extension shares one set of foundations, one roof and one set of drawings. The HomeOwners Alliance puts a standard two storey build at around £1,900 per square metre. You pay more in total, but you get far more space for every pound.
Watch for extra costs on either option:
- Structural calculations and steel beams for large openings
- Kitchen and bathroom fit-out
- VAT at 20%
- Architect and structural engineer fees
- A contingency of around 10% for surprises
On our Kent projects, the biggest budget shocks come from bigger glazing added mid-build. Larger bi-fold doors often mean larger steel beams and fresh structural calculations.
Do You Need Planning Permission for an Extension in Kent?
Often you will not need full planning permission for a single storey extension. Many fall under permitted development rights. A double storey extension almost always needs full planning permission.
Single Storey Extension Planning Rules
Under permitted development, a single storey rear extension can usually extend:
- Up to 3 metres beyond the rear wall for a terraced or semi-detached house
- Up to 4 metres for a detached house
- Up to 6 or 8 metres through the prior approval route
Maximum height is 4 metres, and your materials must match the existing house.
Two Storey Extension Planning Rules
Two storey extensions face tighter limits. Where permitted development applies, the extension must not project more than 3 metres beyond the rear wall. It must also sit at least 7 metres from the rear boundary. In most cases, you will still need full planning permission.
The Kent Catch Most Homeowners Miss
Much of Kent sits on what planners call Article 2(3) designated land. This includes the Kent Downs National Landscape and many village conservation areas. On designated land, the larger single storey permitted development rights do not apply. Listed buildings and Article 4 directions can remove your rights entirely.
We always advise a Lawful Development Certificate, even for permitted development work. It costs about half the standard planning fee. Buyers’ solicitors often ask for it when you sell.
Which Extension Adds More Value, and Which Causes More Disruption?
Going up usually adds more value than going out. An extra bedroom is one of the highest-value improvements to a family home. That makes a double storey extension a strong long-term investment.
Single storey extensions still add value, often around 10 to 15%. Just remember that your street’s ceiling price caps any extension’s value.
Which Is More Disruptive to Live Through?
Living through a single storey build is usually easier. The work hits your garden and ground floor, but you can often stay put. A double storey build brings more scaffolding and weather exposure.
From experience, we leave the first floor knock-through until late in a double storey build. This keeps your upstairs usable for as long as possible.
So, Which Extension Is Right for Your Home?
Your choice comes down to what you need most. Use the checklists below to match an option to your home.
Choose a single storey extension if you:
- Want a bigger kitchen or open-plan living space
- Have a generous garden to give up
- Want a lower cost and a faster build
- Prefer to avoid full planning permission
Go for a double storey extension if you:
- Need extra bedrooms or another bathroom
- Have a smaller garden to protect
- Want the best value per square metre
- Plan to stay in your home for years
How BDC Construction Can Help Across Kent
BDC Construction builds both single and two storey extensions across Maidstone and the wider Kent area. Because we fit both formats, we have no reason to push you one way. Our advice stays honest.
Founded in 2011 by Ben and the BDC team, we bring over 20 years of building experience. We handle every trade, including steelwork and large structural openings. We keep sites clean and partitioned, so you can live at home during the build.
See our recent Kent extension projects to picture the finish. We also explain planning versus building control in plain English. We can introduce architects at the early stage too.
Single or Double: Making the Final Call
The right answer depends on your priorities, not a rule. Pick a single storey extension for lower cost, speed and open-plan living. Pick a double storey extension for bedrooms, value and a smaller garden footprint.
Still weighing it up? Book a free consultation with BDC Construction, and we will recommend the best option for your home. What is holding you back from extending this year?