Site Prep Cost Calculator for Shed & Slab Pads

With the defaults, a 12 by 10 ft shed on a level lot with a gravel pad that extends 1 ft past it on every side comes to $1,186. The pad is 14 by 12 ft, or 168 sq ft. That is $59 to clear the grass, $252 to excavate, grade and compact, $50 for landscape fabric, $107 for 2.4 cu yd of gravel at 4 in deep, $468 for 52 linear ft of timber frame and $250 of mobilization, or about $7.06 per sq ft of pad.

Size changes the cost per sq ft, because mobilization is a flat charge. A 10 by 8 ft shed (a 120 sq ft pad) comes to $981, or $8.18 per sq ft. A 16 by 12 ft shed (252 sq ft) comes to $1,529, or $6.07 per sq ft, and a 24 by 20 ft building (572 sq ft) comes to $2,709, or $4.74 per sq ft.

A small job can hit the floor. Leaving the timber frame off the default pad leaves $718 of work, which the calculator raises to the $800 minimum job charge. Real quotes also depend on access for equipment, the soil, hauling away the spoil and local gravel prices, so use the pricing section to enter figures from a contractor.

A gravel pad is usually the lower-cost way to give a small shed a firm, free-draining base. On the default site the gravel, fabric and timber frame add up to $625. A formed concrete slab priced at $7.50 per sq ft is $1,260 for the same 168 sq ft, which takes the default job to $1,821, or $10.84 per sq ft. That is $635 more than the gravel pad.

Gravel is also easier to adjust later: it can be raked, topped up or levelled again if one corner settles. A slab is more permanent and is often chosen for heavy equipment, a workshop or a building that will stay put for decades. A slab can also be required by a manufacturer's instructions or a local rule, so check before you decide.

The slab line is a single per sq ft price for a 4 in formed slab. A contractor may itemize the base under it, reinforcement and finishing separately, so replace the price in the pricing section with the figure from the quote. To check the concrete volume, use the concrete calculator.

A pad is typically made a little larger than the building so that the walls and the floor edge sit on firm, compacted material rather than right at the edge of the pad, where loose gravel is generally less well supported. The extra width also gives rain running off the roof somewhere to land and drain away, and gives room for the timber frame and for any levelling work around the building.

The margin is the default 1 ft on every side and you can set it from 0 to 3 ft. On the 12 by 10 ft shed, no margin gives a 120 sq ft pad and $981 ($8.18 per sq ft), 1 ft gives 168 sq ft and $1,186 ($7.06), 2 ft gives 224 sq ft and $1,414 ($6.31) and 3 ft gives 288 sq ft and $1,665 ($5.78). The total rises with each foot, but the cost per sq ft falls because mobilization is spread over a larger pad.

Penn State Extension makes a similar point about gravel pads for horse paddocks, saying a shoulder beyond the fence line reduces damage to the edges and that extending the pad beyond the highest traffic zone makes it last longer. A shed is not a paddock and puts far less wear on its pad, but as general practice the same idea carries over: keep the load back from the edge of the gravel.

Slope only changes the excavation and grading line, which the calculator multiplies by 1.2 for a site 8 to 14 in off level and by 1.3 for one 14 to 20 in off level. On the default pad the grading goes from $252 to $302 on a moderate slope and $328 on a steep one, so the total is $1,236 and $1,262. Brush instead of grass raises the clearing line from $59 to $210 (total $1,337), and bare ground drops it to nothing (total $1,127). A steep site can also need retaining walls or engineering that this calculator does not price.

Drainage matters as much as the material. Penn State Extension, writing about gravel pads for paddocks, says the surface must shed rainwater and snowmelt by a crown or a slope, that the subgrade must be highly compacted and that all vegetation, roots and organic matter must be removed before grading. It also recommends topping the compacted subgrade with geotextile fabric before the gravel goes on. Those steps are what the clearing, compaction and fabric lines in the calculator pay for. A paddock pad is built for horses and light vehicles, so its specification, such as a support base at least 6 in thick, is heavier-duty than a shed pad usually needs; use it as a guide to the steps rather than to the depths.

Excavation can hit buried lines, so call 811 before you dig. 811 is the free national before-you-dig service, and the 811 site says to contact them a few business days before any digging so utilities can mark their lines. A permit, which is an optional $100 line in the calculator, may also be needed for the building itself; your local building department can tell you.