When planning an extension, loft conversion, or structural alteration, unexpected problems can quickly increase costs and delay your project.

Below are some of the most common risks homeowners face — and what you can do early on to avoid them.

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1️⃣ Not Knowing What’s Hidden Inside the Existing Structure

⚠️The Risk

Sometimes there isn’t enough information about how your home was originally built. Once work starts, builders may discover:

• Unexpected steel beams
• Hidden structure in poor condition
• Different wall construction
• Hidden structural changes
• Irregular layouts

This can lead to:

• Design changes during construction
• Project delays
• Increased costs

✅How to Reduce the Risk

✔ Carry out site investigations (small openings in walls/ceilings to inspect structure)
✔ Commission a measured dimensional survey so drawings reflect the true building layout

Spending a little early can prevent expensive surprises later.

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2️⃣ Shallow Foundations Requiring Underpinning

⚠️The Risk

Older properties often have shallow foundations (less than 500mm into the ground). If they are not deep enough to suit your new extension, particular in projects where the floors are proposed to be lowered, underpinning may be required.

Underpinning can significantly increase construction costs.

✅How to Reduce the Risk

✔ Dig trial pits early in the design stage to confirm foundation depths

This allows the design to be adjusted before construction begins, minimizing surprises/costs later.

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3️⃣ Discovering a Public Sewer in Your Garden

⚠️The Risk

You may discover a public sewer running across your rear garden during a rear extension construction.

If this happens, you’ll need approval from your Local Water Authority (for example, Thames Water in London).

This can cause:

• Delays due to redesign and longer approval process.
• Abortive works (work that has to be undone to satisty the Local Water Authority’s requirement)

✅How to Reduce the Risk

✔ Carry out an asset search before design – Click here for Thames Water
✔ Undertake a CCTV drain survey – usually cost between £350 – £600

⚠️Important: Asset maps do not always show private or rear-of-property sewers — CCTV surveys provide extra certainty.

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4️⃣ Poor Soil Conditions

⚠️The Risk

If soil conditions are weaker than expected, foundations may need to be changed at the last minute.

This can mean:

• Additional engineering design fees
• Delays due change in design and construction method
• Increased construction costs

✅How to Reduce the Risk

✔ Carry out a soil investigation before finalising the design.

Typical cost: £1,500 – £2,500
This is often money well spent compared to redesigning foundations mid-project. In some cases, you may even make an overall construction cost saving if the soil investigation is more favourable.

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5️⃣ High Water Demand Trees Nearby

⚠️The Risk

High Water Demand trees close to your property (especially on clay soil) can affect foundation depths.

Building Control may require:

• Much deeper foundations
• Or even piling

Both can increase costs and delay the programme.

✅How to Reduce the Risk

✔ Commission a tree survey early in the design process. Costs vary but this can be done between £300 – £800.

This allows foundation design to account for nearby trees from the outset, rather than being asked by Building Control to change later.

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6️⃣ Choosing Inexperienced Contractors

⚠️The Risk

Appointing contractors purely based on the lowest quote can lead to:

• Unsafe work
• Structural damage to your & your neighbour’s property
• Poor workmanship
• Issues with neighbours

✅How to Reduce the Risk

✔ Check previous projects, visit in person if possible
✔ Ask for references
✔ Review track record
✔ Don’t choose on price alone

Quality and experience matter.

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7️⃣ Not Considering the Party Wall Act

⚠️The Risk

If your work affects a shared wall or boundary, you may fall under the Party Wall etc. Act 1996.

Ignoring this can cause:

• Legal disputes
• Delays
• Construction start date being pushed back

✅How to Reduce the Risk

✔ Speak to an experienced Party Wall Surveyor early
✔ Allow time for notices and agreements

This keeps the process smooth and avoids unnecessary disputes down the line.

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8️⃣ Work Being Rejected by Building Control

⚠️The Risk

If Building Control rejects part of the work, you may need to:

• Open up completed work for inspection
• Re-do some of the work to the satisfaction of the Building Control Officer.
• Pay additional fees
• Face delays

✅How to Reduce the Risk

✔ Appoint experienced designers
✔ Use competent contractors that adhere to industry best practices
✔ Consider appointing your original designers to undertake periodic construction inspections for added quality control assurance.
✔ Only start work after receiving Building Control approval

Professional design and proper approval reduce the risk of costly re-work.

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🏁 Final Advice for Homeowners

We get it, undertaking a renovation project can be quite stressfull with lots of things to consider and juggle. Most project risks can be avoided with early investigations and the right professional advice.

The key principles to bear in mind are:

✔ Investigate before you design
✔ Design before you build
✔ Choose experience over cheapest price
✔ Allow time for approvals
✔ Avoid rushing into construction

Spending a little more time and money at the start of your project can save thousands of pounds and months of stress later.