What permeable tarmac is and when it should be used

When porous tarmac is worth paying for, and when a standard drive does the job for less.

Permeable tarmac is an open graded asphalt surface that lets rainwater drain straight through it into the ground beneath, instead of running off into a gully or out onto the road. Standard tarmac does the opposite: it sheds water across the surface to wherever you send it. For most homeowners looking at permeable tarmac in Kent, the decision comes down to two questions. Where is the water going to go, and does the drive sit between your house and the road?

Get those two answers right and the surface choice makes itself. Get them wrong and you either overspend on a build-up you never needed, or end up with a driveway that should have had planning permission behind it.

What permeable tarmac actually is

Permeable tarmac, also sold as porous asphalt or open graded asphalt, is made by leaving most of the fine material out of the mix. What you get is a surface course full of connected voids, so water passes through it rather than sitting on top. It looks coarser than a standard drive up close. From the pavement, most people cannot tell the difference.

The surface is the part everyone talks about. It is not the part doing the work.

Underneath a permeable drive sits a sub-base of clean, angular stone with no fines in it, laid deeper than a standard build-up. That layer is a reservoir. Rain passes through the tarmac, fills the voids in the stone, and soaks away into the subgrade at whatever rate the ground can take. On sites where the ground drains slowly, the sub-base is sized to hold the water back and release it gradually. Lay porous asphalt on a conventional compacted Type 1 base and you have built an expensive puddle, because the water has nowhere to go once it gets through the surface.

That is the single most common mistake we see on drives laid by other contractors. The surface is genuinely permeable. The thing underneath it is not.

Permeable vs standard tarmac at a glance

When standard tarmac is the right call

Plenty of installers will tell you every drive needs to be permeable now. That is not what the rules say, and it is not what most sites need.

The planning trigger only applies to hard surfacing between your house and the highway. Rear gardens, side accesses and anything behind the building line sit outside it entirely. If you are tarmacking a parking area round the back, standard tarmac is fine and you are not asking anyone’s permission.

Even at the front, there is a second route. According to the Planning Portal, planning permission is not required if the rainwater from the drive is directed to a lawn, border or other permeable area to drain naturally within your own property. A standard tarmac drive laid to fall towards a planted border, at any size, stays permitted development. Most driveway blogs skip that clause. It is the cheapest compliant option on a lot of Kent properties, particularly on plots with a decent front garden still intact.

Standard tarmac also wins on:

• Resurfacing. If the existing base is sound and drainage already works, overlaying with standard tarmac is a fraction of the cost of digging out for a permeable build-up.
• Sloping sites where water is already managed. A drive falling to an existing channel drain or an established soakaway does not need reinventing.
• Farm tracks, yards and access roads. Heavy vehicle loading and open graded surfaces are an awkward pairing, and there is usually somewhere for water to go.
• Tight budgets on a big area. A well-laid standard drive beats a permeable one built too thin because the quote had to come down.

We cover all of this at survey stage as part of our domestic driveway installation across Kent, and we will tell you when the cheaper option is the right one.

When permeable tarmac is worth the extra

Permeable tarmac earns its cost when there is genuinely nowhere for the water to go.

The clearest case is a front driveway that runs kerb to house with no soft landscaping left. Nothing to fall towards, no border, no lawn. Over five square metres, which is barely one parking bay, you are choosing between permeable surfacing and a planning application. Permeable is faster, cheaper and less risky than the application.

Ground conditions across our patch push the same way:

• Romney Marsh and the low lying ground south of Ashford. A high water table makes a conventional deep soakaway difficult to sign off and unreliable in a wet winter. A shallower, wider permeable build-up spreads the storage out instead of relying on one point of discharge.
• Clay on the Weald. Around Charing, Bethersden and the villages west of Ashford, the subgrade takes water slowly. A permeable sub-base sized as an attenuation layer holds the storm and releases it at the rate the clay will actually accept.
• Drives that fall towards the house. If the levels send water at your threshold, a surface that takes it away vertically solves a problem that surface drainage keeps having to manage.

There is a resale argument too. A permeable drive is documented as permitted development. A large impermeable one laid without permission is a question on a conveyancing form, and unpicking it later costs more than laying it properly did.

Maintenance and lifespan

Here is the part the sales pitch tends to leave out. Permeable tarmac silts up.

Fine material works into the voids over time: windblown soil, leaf litter breaking down, mud off tyres, sand and grit tracked in from the road. As the voids fill, infiltration slows. A neglected porous drive can end up shedding water like a standard one within a few years, which means you paid for a permeable surface and no longer have one.

Keeping it working is not difficult, and will save you a lot of hassle later down the line:

• Sweep or blow leaves and debris off, particularly through autumn.
• Keep loose material away from the edges. Bark mulch, gravel borders and open soil beds next to a permeable drive are the main culprits.
• Have the surface vacuum swept or gently jetted periodically to draw silt back out of the voids. Frequency depends on the site, not the calendar. A drive under a line of limes needs it far more often than an open one.
• Do not seal it. Sealants close the voids, which defeats the entire point.

Standard tarmac asks less of you: keep the edges supported, clear the gullies, deal with weeds at the joints, and watch for oil spotting, which softens the binder.

On lifespan, both surfaces live or die on what is underneath them. A properly excavated, properly compacted base laid to the right depth for the loading will outlast a thicker surface course over a base that was rushed. Failures we get called out to are almost always base failures showing up at the surface, not the tarmac itself giving up. If you want a straight answer on expected life for your specific site and traffic, ask at survey and we will talk through the build-up for your drive rather than quote a number that means nothing without context.

FAQs

Do you need planning permission for a tarmac driveway in Kent?

You need planning permission for a front driveway over five square metres laid in standard impermeable tarmac, unless the rainwater is directed to a permeable area within your own property, such as a lawn or border. Permeable surfacing needs no permission at any size. Rear and side drives are not covered by the rule. The rules apply across England, Kent included.

Can permeable tarmac be laid over an existing driveway?

Not usefully. Laying porous asphalt over an existing impermeable base gives you a surface that water passes through and then has nowhere to go, which causes water to sit in the surface course and break it up in frost. A genuine permeable drive needs excavating out and rebuilding with an open graded sub-base. If your existing base is sound and you want to save money, resurfacing in standard tarmac is the better use of the budget.

Does permeable tarmac work on clay soil?

Yes, with the sub-base designed for it. Clay accepts water slowly, so the open graded layer beneath the surface is sized to store the storm and release it at the rate the ground can take. On very heavy clay the design may also include a controlled outfall. This is a site-by-site calculation, not a standard detail, and it is why clay sites need a proper survey before quoting.

Is permeable tarmac more expensive than standard tarmac?

Yes. Most of the difference is not the tarmac itself but the groundworks: a deeper excavation, more spoil carted away, and more stone going back in. The gap widens on sites with difficult access or where the subgrade needs additional work. Against that, weigh the cost and delay of a planning application if permeable is what keeps you inside permitted development.

Does permeable tarmac get damaged by frost?

Not when it is built correctly, because water drains out of the structure rather than sitting in it. Frost damage on porous surfaces happens where drainage has failed, either because the sub-base was wrong from the start or because the voids have silted up and water is now held in the surface course. Keeping it clean is the frost protection.

Are the driveway drainage rules going to change?

The five square metre rule has been in place since October 2008 and still stands. Schedule 3 of the Flood and Water Management Act 2010, which would make sustainable drainage systems mandatory on new developments in England, has never been commenced, though the National Planning Policy Framework was updated in December 2024 to push delivery of sustainable drainage. Nothing currently changes the position for a domestic driveway, but the direction of travel is towards more drainage control, not less.

Not sure which one your drive needs?

Send us the address and a photo of the front of the property. We will tell you whether your site needs permeable tarmac or whether standard tarmac with the right falls does the job for less, before anyone quotes you for anything.