Is the business outgrowing its building? Stock in the walkways, machinery squeezed in wherever it fits, staff sharing desks that were never meant to be shared. Before you start looking for a new site, it’s worth asking whether you can extend the one you’ve got. More often than not you can, and it’s usually quicker and cheaper than moving.
Extensions are a big part of what we do at Hamilton Steel Buildings. This guide pulls together what we’ve learned from putting steel extensions onto all sorts of existing buildings, from food warehouses in South Shields to engineering units in Leeds.
Signs It’s Time to Extend
There’s no single right moment, but these are the reasons clients usually give us:
- New machinery that won’t fit. Shalam Thermoforming in Sunderland needed a 21m x 14m extension built around a new machine, and Thorpe Design & Technologies needed room for a new spray booth.
- More staff. Simply Food Solutions had grown to the point where they needed new office space. Rather than move, their MD asked us to extend the existing building.
- Not enough storage. Kalma Foods needed more storage at their premises, so we extended an existing mono pitch building by 19m x 9.5m.
- Spare land doing nothing. DAS Engineering in Leeds had unused land next to their unit and wanted to get the most out of it.
- Paying for space elsewhere. If you’re renting overflow storage, that money could be going into a building you own. We’ve done the sums in the benefits of steel storage buildings.
Why Steel Works Well for Extensions
A steel frame can be bolted to an existing steel building or built alongside a brick or concrete one, and the parts are made off site to the exact sizes on the drawings. That’s what makes it a good fit for extensions:
- Less disruption. Most of the work happens in a factory, so the time we spend on your site is short. That matters when you’re still trying to run a business next door.
- Open space. A portal frame gives you clear span with no posts in the middle, which suits warehouses, workshops and production space.
- Lighter foundations. A cold rolled steel frame is light, so foundations are usually smaller than for a masonry extension of the same size.
- Room to grow again. A steel extension can itself be extended later. Durham Animal Feeds came back to us to extend a building we’d put up for them a few years before.
- Low maintenance. No rot, no warping and nothing for pests, so the extension won’t need much looking after.
Check What Land You Own and What You’re Allowed to Do
It sounds obvious, but before any design work starts, be sure exactly where your boundary is and on what terms you hold the property.
- Freehold gives you the most flexibility.
- Leasehold almost always means you need the landlord’s consent, and some commercial leases ban extensions or alterations altogether. Get that consent in writing before you spend money on drawings.
- Restrictive covenants can limit building on part of a site, cap the size of buildings or protect a neighbour’s light. If you’re not sure, a property solicitor can check the title for you.
Planning Permission and Building Control
Some commercial and agricultural extensions fall under permitted development, depending on size, position and how the site is used now. Plenty don’t. We’ve gone through the rules in more detail in do you need planning permission for a steel building.
A couple of tips from experience. Many councils offer a pre-application service where you can get feedback on a proposal before submitting it properly, which can save a lot of back and forth. And Building Control approval is a separate thing from planning that almost always applies. We supply the GA drawings and structural calculations for it free of charge, and we can handle the planning application and deal with the council for you as part of our project management.
Joining the New Building to the Old One
This is the part that makes an extension harder than a standalone building. The new frame has to meet the existing one cleanly and stay watertight at the joint for years. Some of the ways we’ve handled it:
- For Thorpe Design, our design team made bespoke flashings so the new duo pitch extension tied into the existing cladding without ugly joins.
- For Shalam Thermoforming, we designed and fitted a bespoke weather seal where the new hot rolled extension met the old building.
- At Mixed Freight Services near Heathrow, the new 24.5m x 20m building connected to the existing one on three sides, with a 20 degree roof pitch to give more storage height inside.
- At Fairfield Golf Club, the extension came off the gable end. Because it shared a boundary with a housing estate we used composite cladding to form a fire boundary wall.
Matching colours and profiles to the existing building also makes a big difference to how the finished job looks. Cladding comes in a wide range of colours (most of our jobs are Goosewing Grey or Juniper Green), which helps when you’re matching an older building. See our steel cladding page for options.
Groundworks Next to an Existing Building
Building right up against an existing structure often turns up things you wouldn’t find on an open field. A few real examples:
- At Shalam, the existing building’s foundations ran into the footprint of the new one, so our engineers designed a structural ring beam to work around them.
- For St Astier in Seaham, a sewer ran alongside the extension. After a site survey with Northumbrian Water, our team worked through the night to power float the floor slab.
- At DAS Engineering in Leeds there was a gas pipe running through where the slab needed to go. We worked with the gas network to locate and protect it.
- For John Clark Valves in Stockton, the ground conditions meant a ring beam and raft foundation instead of the usual pads and 150mm slab, so the building and foundations settle as one.
We do groundworks in-house, so whoever designs the base is also responsible for the steel that sits on it. There’s more on this in steel building foundations explained.
Keeping the Business Running During the Build
Most clients can’t shut down while we work. St Astier’s site runs seven days a week, so the work had to be planned around that. At Free Spirit Automotive the workshop stayed open throughout, and we worked with engineers and Building Control to meet the fire and structural requirements. For Oil Salvage in Liverpool we coordinated the groundworks, pad foundations and steel erection on a live site to a tight programme.
Planning access, deliveries and working areas properly at the start is what makes this possible.
What Does a Steel Extension Cost?
It depends mostly on size, eaves height, cladding, doors and above all the ground. We’ve written a full breakdown in how much does a steel frame extension cost in 2026.
Talk to Us About Extending
For most projects we’ll visit the site, look at the existing building and the ground, and come back with a 3D design and a quote. You’ll have one project manager from first visit to handover.
Request a quote or call 0191 359 2339. You can see more of our extension work in our case studies.