Modular Home Foundations Explained: Slab, Strip or Screw Piles

A modular home gets built in a factory, but its foundation does not. The foundation is a separate groundworks job, designed for your specific plot by a structural engineer, poured or installed on site, and signed off by Building Control before the modules ever arrive on a lorry. The factory builds the box to a known weight and a known set of fixing points; the groundworks contractor builds something level, permanent and strong enough to carry that box on your particular ground. Get the two to meet cleanly and craning the modules into place takes a day. Get the foundation wrong and you have an expensive, immovable problem.

For a habitable modular dwelling in the UK, three foundation types do almost all the work: a concrete slab or raft, strip (or trench-fill) footings, and engineered screw piles. This explains how each one works, what it costs in rough terms, the ground it suits, and the one distinction that trips up homeowners more than any other: cheap garden-room ground screws are not the same thing as engineered screw piles, and you cannot put a house on the former.

Do modular homes need foundations at all?

Yes, and the same ones a brick house needs. A modular home is a permanent building under the Building Regulations, not a caravan or a park home. It has to sit on a permanent, code-compliant foundation designed from a site-specific ground investigation. The structure side is governed by Building Regulations Approved Document A, with the detailed depths and details guided by the NHBC Standards and warranty providers’ technical manuals.

The sequence usually runs like this:

• A soil and ground investigation tells the engineer what you are building on.
• A structural engineer designs the foundation for that ground and for the module weights and fixing points the manufacturer supplies.
• Building Control checks and signs off the design, then inspects the work.
• A structural warranty provider (LABC Warranty and similar) sets its own technical requirements, because most mortgage lenders will not lend without a valid structural warranty in place.

That last point matters more than people expect. If you are financing the build, the foundation has to satisfy not just Building Control but the warranty provider too, which is one reason an unverified, uncertified foundation is a false economy. If you are weighing up the whole route to a finished home, our buying a modular home UK guide walks through where groundworks sit in the wider process.

The three realistic UK choices

1. Concrete slab or raft

A raft is a single thick layer of reinforced concrete that spreads the building’s load across its entire footprint, like a tray sitting on the ground. For a modular home on flat, stable, well-drained ground, it is the default for good reason: it gives you a level, ready platform the modules sit straight onto.

Typical specification for a domestic raft:

• Slab thickness commonly in the 200 to 300mm range, with edge beams thickened where loads concentrate. Overall raft depths vary widely with load and soil.
• Reinforced concrete, often specified around RC32 (C32/40), rising to RC35 or RC40 where edge-beam loads are significant.
• A compacted sub-base, commonly around 150mm of Type 1 MOT.
• A 1200-gauge polythene damp-proof membrane beneath the slab.
• Around 50mm minimum cover to the reinforcement.

Rough cost: laying a concrete slab tends to fall in a per-square-metre band, with the labour element a portion of that and ready-mix concrete supplied by the cubic metre on top. Because cost scales with footprint, a larger plan means a larger bill. Timeline is usually one to two weeks including the sub-base, formwork, pour and cure, weather depending.

Best for: flat, stable, well-drained sites where you want a complete platform and have no awkward access or slope to fight.

2. Strip (or trench-fill) footings

Strip footings are continuous reinforced concrete strips running under the load-bearing walls, rather than under the whole footprint. The strip width is generally two to three times the width of the wall it carries. Trench fill is the same idea taken to its logical end: instead of a thin strip at the base of a deep trench, you fill most of the trench with concrete, which is faster on labour but uses more concrete.

In most UK conditions the minimum foundation depth is around a metre, and many domestic trenches land somewhere between 900 and 1200mm to get below the zone affected by frost and seasonal moisture movement. On shrinkable clay near trees the required depth climbs sharply (see the soil section below).

Rough cost is quoted per linear metre of trench rather than per square metre, so the price is driven by the perimeter and internal wall runs, not the floor area. Trench fill runs higher per metre than a standard strip because of the extra concrete.

Best for: ground where the firm, load-bearing layer is within reach of a normal trench, and where you would otherwise be building over made ground only near the surface. Strip footings are traditional, well understood by every groundworker, and easy for Building Control to accept.

3. Screw piles

A screw pile is a steel shaft with one or more helical (screw-shaped) plates near the bottom. A machine winds it into the ground until it reaches competent soil at depth. The load is carried on the end-bearing helix, deep down, not on the ground at the surface.

The advantages that matter for a modular build:

• Speed. Piles go in fast, often in minutes each, and a small house plot can be done in a day. There is no concrete to cure, so the structure can be loaded straight away and the modules can follow without a wait.
• Difficult ground. Piles can be driven deeper to reach a firm layer, bypassing soft, made or variable ground near the surface. They suit sloping plots, flood-prone sites and poor ground that would force a strip foundation very deep.
• Less spoil and disturbance. No large trench dig, far less muck away, and a smaller footprint of disruption on a tight site.

Load is not guessed. Installation is torque-controlled, with the torque correlated to the design load, and on-site load testing is carried out where the engineer requires it. The actual capacity of a pile depends on the soil, the shaft and helix size and the depth, which is why no honest supplier quotes a single universal load figure.

Rough cost is typically quoted per metre of pile, in a similar broad band to strip footings per linear metre, varying with soil, depth and the machinery needed. You can also value-engineer a hybrid: ground beams or a strip foundation can be designed to sit on a grid of screw piles, giving you a conventional bearing surface for the modules on top of a piled solution underneath.

Building Control and warranty providers increasingly accept screw piles, but on conditions: supporting engineering calculations, load testing where specified, and manufacturer certification (UKCA or CE marking). Some systems hold an LABC Registered Detail, which makes sign-off smoother. For an authoritative reference on what Building Control expects from foundations generally, the Planning Portal guidance on foundations is the official starting point.

The distinction that catches people out: ground screws vs screw piles

This is the single most important practical warning in this article. “Ground screw” and “screw pile” get used interchangeably online, and they are not the same product or the same engineering.

Ground screws are the garden-room, decking and lightweight-structure grade. The load is carried by friction along the threaded shaft, often after drilling a pilot hole. Because the load is held along the surface of the screw, ground screws are more vulnerable to frost heave and seasonal moisture movement: the ground grips them and can lift them. They are fine for a studio at the bottom of the garden. They are not a house foundation.

Screw piles carry the load on the end-bearing helix plate, set deep enough that it sits below the heave zone the vast majority of the time. That changes the engineering completely. The capacity per pile is far higher: as a rough illustration, around nine helical piles can do the work of roughly twenty ground screws on a small to medium garden room. Scale that to the weight of habitable modules and the difference is the whole point.

UK Helix has a clear technical explainer on the difference between DIY ground screws and screw piles that is worth reading before any salesperson tells you a garden-room kit will do for your house.

Several genuine engineered-pile and ground-screw brands operate in the UK, including Krinner, Stop Digging, No More Digging, GroundPlug, GoliathTech, Ground Sun, Radix, ABC Anchors and Aarsleff. The point is not the badge; it is that for a home you want a calculated, torque-verified, certified pile system, installed by a contractor who can hand Building Control and your warranty provider the paperwork.

If you are tempted to buy a consumer ground-screw kit online for a habitable build, do not. Those kits are sold for garden rooms and decking; you can check current prices for that kind of project all you like, but they are not a substitute for an engineered pile foundation under a modular house.

Ground conditions decide more than budget does

The foundation is chosen for the ground first and the budget second. A few situations push the decision hard.

Shrinkable clay near trees. This is the classic UK foundation deepener. Clay shrinks as nearby trees draw out moisture, and footings have to reach below that active zone. The NHBC Standards Chapter 4.2 covers building near trees on shrinkable soils: depths can run from around a metre to two and a half metres or more, and you should consult the tables whenever a tree sits within roughly 1.25 times its mature height of the foundation. Large, thirsty species such as oak or poplar on high-shrinkage London Clay can push depths well beyond two and a half metres. On a plot like that, a deep strip becomes expensive and slow, and screw piles that reach competent strata below the active clay often make more sense.

Sloping plots. A slab wants a level platform, which on a slope means cut and fill or stepped foundations. Screw piles handle level changes more gracefully because each pile is set to its own depth and the beams above bring everything to a level plane.

Flood-prone or poor/made ground. Where the surface ground is soft or unreliable, piles drive through to something firm. A slab or strip would either need to go very deep or sit on ground engineering it cannot safely rely on.

Flat, stable, well-drained ground with good access. This is where a raft slab is hard to beat: simple, level, complete, and familiar to everyone signing it off.

As a budgeting reality, foundations can swallow up to a third of a build budget on a difficult plot, which is why the ground investigation at the start is money well spent rather than a tick-box.

So which one for your modular home?

There is no single best foundation; there is the right one for your ground, slope, access, speed and budget. A working decision framework:

• Flat, stable, dry, easy access, no rush: concrete slab or raft. Complete platform, well understood, scales with footprint.
• Firm layer within trench reach, traditional groundworks preferred: strip or trench-fill footings.
• Sloping, flood-prone, made/poor ground, near trees, or you need speed: engineered screw piles, with calculations, load testing and certification.
• Want a conventional bearing surface but the ground is poor: ground beams or a strip designed to sit on screw piles.

The cost difference between these is real but it is rarely the deciding factor, because the ground often rules out the cheaper option before you get to compare prices. For how foundation choice feeds into the total, see our breakdown of 3-bedroom prefab house cost and the wider question of whether modular homes are cheaper than brick. Foundations are also one of the places where the modular versus traditional build comparison narrows, because the groundworks are essentially the same job whichever way you build above ground.

Who designs it and who signs it off

To keep the responsibilities clear:

• The module manufacturer supplies the module weights, fixing points and the drawings the engineer needs.
• A structural engineer designs the foundation from the ground investigation and those loads.
• Building Control approves the design and inspects the work on site.
• A structural warranty provider (such as LABC Warranty) sets its own technical conditions, and most mortgage lenders require that warranty before they will lend.

A modular warranty package commonly includes long structural cover, shorter product cover and a defects period in the first year, but the structural warranty that lenders care about hangs on the foundation being designed, built and certified properly. That is the thread tying all of this together: the foundation is where the engineering, the regulations, the warranty and the mortgage all meet.

Frequently asked questions

Do modular homes need foundations? Yes. A modular home is a permanent building, so it needs a permanent, code-compliant foundation designed for your specific ground and signed off by Building Control, exactly like a brick house. The factory builds the modules; the foundation is a separate site-specific groundworks job.

How deep do modular home foundations need to be? In most UK conditions, around a metre, with many domestic trenches landing between 900 and 1200mm to clear frost and seasonal moisture movement. On shrinkable clay near trees the depth can rise to two and a half metres or more, following the NHBC Standards tables for building near trees.

What is the best foundation for a modular home? There isn’t one universal best. A concrete slab or raft suits flat, stable, well-drained ground; strip or trench-fill footings suit ground with a firm layer within trench reach; engineered screw piles suit sloping, flood-prone, made or poor ground, sites near trees, and builds that need speed. The ground conditions usually decide before budget does.

Are screw piles strong enough for a house, not just a garden room? Yes, provided they are engineered screw piles, not garden-room ground screws. Engineered piles carry load on a deep end-bearing helix, their installation is torque-controlled and correlated to the design load, and they are load-tested where the engineer requires it. Capacity depends on soil, helix and shaft size and depth, which is why no reputable supplier quotes a single universal figure.

Can I use cheap DIY ground screws for a modular home? No. Ground screws carry load by friction along the shaft and are more vulnerable to frost heave and moisture movement; they are made for garden rooms, decking and lightweight structures. A habitable home needs engineered, certified screw piles with calculations and load testing behind them.

Will Building Control and my mortgage lender accept screw piles? Increasingly, yes, but on conditions. Building Control wants supporting engineering calculations, load testing where specified, and manufacturer certification (UKCA or CE marking); some systems hold an LABC Registered Detail that smooths the process. Lenders almost always require a valid structural warranty, so the pile system has to satisfy the warranty provider’s technical requirements too.

Which foundation is fastest to build? Screw piles. They install in a day on a small plot, often minutes per pile, with no concrete to cure, so the modules can follow immediately. A slab typically takes one to two weeks including sub-base and cure, and strip footings need the trench dug and the concrete cured before work continues.

Who designs and approves a modular home foundation? A structural engineer designs it from a site-specific ground investigation and the manufacturer’s module loads. Building Control approves the design and inspects the work, and a structural warranty provider sets its own technical conditions on top, which is what lenders rely on.