External works are almost always the last section of an estimate and almost always the first to be cut. Paving, drainage, boundaries, planting: none of it is in the building, all of it is visible from the street, and it is the part of the job the client walks across every day.
The trouble is that externals are measured differently from everything inside the building. They start with levels, they are built on ground that somebody else left in a state, and a large share of the cost is in the ground rather than on it.
This guide covers the order of measurement for a domestic scheme: levels, excavation, sub-base, paving, edges, drainage, boundaries, planting. It ends with a full worked example, because externals are the section where a worked example teaches more than a list of rules.
Levels come before quantities
Before measuring a single square metre of paving, establish three things:
The finished floor level of the building
The existing ground level around it
The level of the road, the drain or the boundary you must connect to or fall away from
The building should sit above the surrounding ground, and the paving should fall away from the building at roughly 1 in 60 to 1 in 80 for a paved area. A patio that falls towards the house is not a paving problem, it is a damp problem that shows up two winters later.
That level survey also decides earthworks: whether spoil is being removed, brought in, or moved from one side of the garden to the other. On a sloping site, the difference between those three can exceed the cost of the paving.
Excavation and sub-base
Paving build up is a stack of layers, and the depth of each one sets the excavation:
Layer
Typical thickness
|---|---|
Paving units
50 to 80 mm
Bedding
30 to 40 mm
Sub-base
100 to 250 mm, depending on use
Geotextile
1 mm
A domestic footpath needs 100 mm of sub-base. A driveway for a car needs 150 to 200 mm. A driveway where a delivery lorry might park needs 250 mm or more, and the paving itself needs to be a block or a heavier slab.
Excavation volume is area multiplied by total build up, plus 50 mm for a soft spot or an over dig. Then the spoil: it bulks by 20 to 30 percent when removed, and on a garden it can often be spread rather than taken away, which is a large saving.
Sub-base is ordered by volume, and it is compacted in layers of no more than 150 mm, so a 200 mm sub-base is two passes with a plate or a roller. Compaction is plant time and it is not optional: an uncompacted driveway settles into ruts within a season.
Paving quantities, and the waste that depends on the pattern
Blocks and slabs are ordered by area plus a waste that depends on the laying pattern and the shape of the area:
Layout
Waste
|---|---|
Slabs, straight, square area
5 to 7 percent
Blocks, 90 degree herringbone
10 percent
Blocks, 45 degree herringbone
12 to 15 percent
Any pattern with curved edges
15 to 20 percent
Cut borders and banding
Add 10 percent on top
A 40 m² patio in slabs with a curved edge can need 47 m² of material, and the 7 m² that never gets laid is not waste in the estimating sense, it is the price of the shape.
Slab size matters too. A 600 x 900 slab has more perimeter per square metre than a 600 x 600, so cuts cost more. Large format slabs also need a lifting frame or two people, which changes the labour rate.
Bedding, joints and the detail that fails first
Bedding is typically 30 to 40 mm of sharp sand, or a mortar bed for some slab types. The material quantity is the paving area multiplied by the bed thickness, plus 15 percent because bedding compacts and is lost at the edges.
Jointing is either kiln dried sand brushed into the joints, a polymeric compound, or a mortar joint for slabs. Each is measured differently:
Kiln dried sand: about 1 to 2 kg per square metre for blocks
Polymeric jointing: sold by weight, coverage per square metre stated by the maker
Mortar joints: measured in linear metres of joint, which on a slab patio is a surprising amount
The joint is what fails first in a patio. A joint that is not full lets water into the bedding, the bedding washes out, and the slab rocks. It is the cheapest item on the list and the most important.
Edge restraint, the item that holds everything together
Block paving without edge restraint spreads. The blocks creep outwards, the joints open, and after two winters the surface is wavy.
Edging is a linear item:
Measure the perimeter of the paved area
Add kerbs, edging blocks or a concrete haunch
Count the haunch as a concrete volume along the same length
Add the corners, which are cut units and slower
For a 40 m² patio, the perimeter might be 26 m, and the edging, haunch and cuts are 8 to 12 percent of the total paving cost. A quote that omits the edging is a quote for a surface that will not stay flat.
Steps, retaining walls and level changes
Any change of level introduces a set of items that are measured on their own:
Steps: counted, with treads, risers and a foundation
Retaining walls: measured as wall area, plus foundation, drainage and a coping
Ramps: measured as area with a gradient that affects the sub-base
Balustrades and handrails: linear, and required above certain heights
A retaining wall over about 600 mm usually needs engineering, and any wall retaining ground next to a building needs a drainage layer and a weep path. Water behind a wall is what pushes it over, and the drainage is the item that gets left off.
Drainage in the garden
External drainage is often the largest single external cost and the least visible:
Gullies and channel drains, counted as units
Pipe runs, measured in linear metres with the fall
Inspection chambers, counted, with a cover and frame
A soakaway or an attenuation crate system, measured by volume
Connections to the sewer or the surface water system, with the fee that goes with them
Two rules worth stating. Surface water should not be connected to the foul sewer where a separate system exists, and the connection fee is a real cost that a client often assumes is free.
Soakaways are sized from the roof and paving area they serve and the permeability of the ground. A trial hole and a percolation test decide the volume, and guessing it produces either a puddle or an unnecessary excavation.
Boundaries, planting and the finish
Fencing: linear metres of panel, posts at 1.8 to 2.4 m centres, gravel boards, and a gate or two
Topsoil: by volume, at the depth specified, and it should be tested rather than assumed
Turf or seed: by area, with a water allowance for establishment
Planting: counted, with a plant size and a tree pit detail
Lighting: counted points, with cable and a transformer
External sockets and taps: counted, with cable and pipe runs
Planting is the section where the estimate and the reality diverge most. A planting plan with 40 shrubs, 4 trees and a hedge run is a quantity. An allowance of a few hundred against a full garden is not.
A worked example
A rear garden for a house refurbishment: 40 m² patio, 25 m² path, 18 m of boundary fence, one soakaway serving 55 m² of roof, and 60 m² of turf.
Patio excavation: 40 x (0.06 paving + 0.04 bedding + 0.15 sub-base + 0.05 over dig) = 40 x 0.30 = 12 m³
Path excavation: 25 x (0.05 + 0.03 + 0.10 + 0.05) = 25 x 0.23 = 5.75 m³
Total spoil: 17.75 m³, bulked at 1.25 = 22 m³, but 8 m³ can be spread in borders, so 14 m³ removed
Sub-base: 40 x 0.15 = 6 m³ plus 10 percent = 6.6 m³, and 25 x 0.10 = 2.5 m³ plus 10 percent = 2.75 m³
Slabs: 40 m² plus 12 percent for the curved edge = 45 m²
Blocks for the path: 25 m² plus 10 percent herringbone = 27.5 m²
Bedding sand: 65 m² x 0.04 = 2.6 m³ plus 15 percent = 3 m³
Edging: patio perimeter 26 m, path perimeter 30 m, total 56 m, plus 10 percent = 62 m with haunch concrete at 0.05 m³ per metre = 3.1 m³
Jointing sand: 65 m² at 2 kg = 130 kg
Soakaway: 55 m² of roof, at a notional 0.1 m³ of storage per m² in a suitable soil, but the percolation test decides
Fence: 18 m, so 11 posts and 10 panels, plus 2 bags of Postcrete per post
Turf: 60 m² plus 5 percent = 63 m², with topsoil at 100 mm = 6 m³
Every one of those lines has a supplier and a delivery. Externals are not one order, they are nine.
Protecting what survives, and reinstating what does not
On a refurbishment, the externals exist before the work starts and they will be damaged by it.
Two lines belong in the estimate. Protection, meaning boards over paving where the barrow route runs, and sheeting over borders. And reinstatement, meaning replacing broken slabs, re-levelling a path the machine tracked across, and re-seeding where the scaffold stood.
The cheapest moment to price reinstatement is at tender, as an allowance. The most expensive moment is at the end, when the client looks at the garden and the contractor has already been paid.
Productivity
For planning rather than pricing: two people lay 15 to 25 m² of block paving a day including the cuts, 20 to 30 m² of slabs, and 40 to 60 m² of turf. Sub-base at 150 mm takes two people about 2 hours per 20 m² including compaction.
Fencing is 8 to 12 panels a day in easy ground, half that in clay with stones in it.
Common errors
No level survey, so the paving falls the wrong way
Sub-base thickness chosen for foot traffic then used as a driveway
Edging omitted, so the paving creeps
Spoil removal ignored, or assumed to be free to dispose of
Soakaway sized without a percolation test
Planting priced as an allowance against a detailed plan
No protection for existing surfaces during the build
Drainage connection fee missing
Frequently asked questions
How thick should a driveway sub-base be? 150 to 200 mm compacted for cars, 250 mm or more for occasional heavy vehicles.
How much waste on paving? 5 to 7 percent for straight slabs, 10 to 15 percent for herringbone, and 15 to 20 percent for curved edges.
Do I need edge restraint? For block paving, always. For slabs on a rigid bed, a kerb or haunch improves both appearance and performance.
What fall should a patio have? About 1 in 60 to 1 in 80 away from the building, with the surface finishing below the damp proof course.
How big does a soakaway need to be? It depends on the area draining and the soil permeability. A percolation test is the only reliable way to size it.
How much spoil will a patio produce? Area multiplied by total build up, plus 20 to 30 percent for bulking when removed.
Design the externals in FRELUX
The FRELUX external works calculator takes areas, build up depths and boundary lengths, then returns excavation, sub-base, paving, edging, drainage and planting with costs from your market's price book.