How To Avoid Structural Issues In Home Extensions with Sam Dean | 118

Architecture Business Club host Jon Clayton interviews structural engineer Sam Dean of Porthouse Dean about common structural design pitfalls in home extensions and how to avoid spiraling costs. They discuss ground conditions as a major uncertainty (especially clay), the influence of nearby large trees and desiccation, and the use of low-cost desktop geological reports based on British Geological Survey borehole data to flag risk. They cover ceiling downstands and why beam position is often a cost-and-aesthetics decision between homeowner, architect, and builder, with installation complexity increasing when first-floor joists run into the beam. Sam explains cantilever “rules of thumb” and how corner bifold-door cantilevers can drive up steel and foundation demands, sometimes requiring columns and large foundations due to uplift forces. They address adding an extra storey and the case for trial holes. They also highlight risks of building onto existing, undocumented steelwork from previous extensions, which can force intrusive investigation or replacement when later loft conversions are planned. Sam explains how the architectural design can affect structural costs and outlines what to expect from a good structural engineering service. They touch on AI-generated architectural information, Sam’s launch of an AI review service, and he shares the software tool his business can’t work without.

Today’s Guest

Sam Dean. He started out as a materials scientist and structural engineer, spent a year in the nuclear industry, then teamed up with his friend Chris Porthouse to start PorthouseDean structural engineering. Sam then got hooked on building business systems and automations - to cut out the boring stuff and let his team do better work. When he’s not solving process problems – he’s cycling to work, playing and watching football, or baking crusty bread and homemade pizzas.

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Episode Highlights

00:00 Introduction

00:39 Introducing Sam Dean

01:38 Managing Risk with Groundworks

03:08 The Clay Problem

04:08 Trees Near Extensions: The Hidden Foundation Cost Driver

04:30 When Is a Site Investigation Worth It? Practical Triggers

05:10 Low-Cost Desktop Geology Reports: A Smart Early Warning

05:49 Designers Missing Key Site Info (Like Trees)

06:59 Case Study: The 20m Oak That Shows Up Too Late

07:55 Using Maps + Clay Likelihood to Spot Risk Early

08:48 Removed Trees Still Matter: Clay Desiccation Explained

10:32 Ceiling Downstands vs Flush Beams: Set Expectations Early

11:33 “Where Do I Put the Beam?” Why Engineers Don’t Always Decide

12:13 Joist Direction Changes Everything (and Can Add Thousands)

13:01 Goalpost Frames & Rear Wall Openings: What’s Cost-Neutral?

14:00 Builder vs Client vs Architect: Who’s Steering the Decision?

14:37 Protecting the Homeowner: Budget Trade-Offs in Plain English

15:36 When Architects Aren’t On Site: How Design Intent Gets Lost

18:06 Roles, Responsibility & the Principal Designer Confusion

19:38 Why Small Projects Are So Cost-Driven (and Getting Worse)

21:07 Cantilevers 101: The Rule of Thumb That Saves Your Budget

23:12 Corner Bifolds + Floating Roofs: The Cantilever Trap

25:23 Engineering Workarounds: Columns, Anchors & Uplift Forces

27:34 Adding a Storey: Foundation Reality Checks

29:32 Building on Existing Steelwork: The Missing Calculations Problem

33:37 Prevention Playbook: Trial Holes, Checks, and Lightweight Options

36:46 Quick Wins to Avoid Spiraling Costs (Wind Posts, Pillars, Layout)

41:45 What Great Structural Engineering Service Looks Like

46:49 The Rise of AI

48:55 The One Piece of Software Sam Can't Live Without

50:18 Final Thoughts

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Key Takeaways

Check the Ground Early to Avoid Big Surprises

Learn what's under the ground before you start building. Clay soil can be a big problem. If there are large trees near your building site (especially within 20 metres), they can make the situation worse. The tree roots dry out the clay, which means you might need to dig much deeper foundations. This can cost thousands extra. Even if you remove a tree, the clay takes about three years to go back to normal. You can get a cheap report to check if clay is likely on your site or dig trial holes, which can help you plan ahead.

Think About Where Beams Go Before You Build

Help your client decide if they want the steel beam to show below the ceiling or to hide it inside the ceiling. If you hide it and the floor joists run into the beam, the builder may need to cut the joists and fix them to the side of the beam. This may cost more money. You should talk about this early with your client and builder. It's a trade-off between how it looks and how much it’ll cost.

Plan Cantilevers Carefully to Keep Costs Down

A cantilever is when part of your building sticks out without support underneath. There's a simple rule of thumb: if you want one metre sticking out, you need two metres anchored back inside the building. If you don't follow this rule, you might need heavier, more expensive beams. Corner bifold doors with cantilevers look great, but to keep costs reasonable, make one side shorter (say 1 to 1.5 metres) with the other side being longer (up to 3 metres for example).

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