Should I do this job myself or pay someone?

The usual way people answer this is by asking whether they could manage it, which is the least useful of the five tests. Plenty of notifiable work is technically easy. The questions that matter are about what happens when it is wrong, who has to certify it, and whether the mistake is one you will ever see.

Safety

Step by step

1. Ask first whether the work is legally restricted.Gas work requires a registered engineer. Certain electrical work is notifiable and must be certified. Work to a listed building requires consent. Removing a load-bearing element requires building control and an engineer's design. These are not skill questions.
2. Ask what happens if it is wrong.A badly hung shelf falls down. A badly built retaining wall falls on someone. Work at height, structural work, anything involving fire escape and anything involving asbestos sit in a category where the downside is not financial.
3. Ask whether it needs certification you cannot issue.If the answer is a competent person scheme certificate in England or Wales, an equivalent certification elsewhere in the UK, or a building control completion certificate, then either a registered trade does it, or you make a building control application and have your own work inspected. Doing it uncertified stores up a problem for the sale.
4. Ask whether a mistake would be hidden.Buried work — waterproofing behind tiles, a damp proof membrane, insulation continuity, a pipe joint in a floor void — fails invisibly and reveals itself as damage months later. Visible work fails visibly and can be redone.
5. Ask whether it is on the critical path.A job that stops everything else if it takes three weekends longer than expected has a hidden cost. Doing your own decorating at the end is very different from doing your own first fix in the middle.
6. Price your own time honestly.Including the tool hire, the wasted materials of the learning curve, the second trip to the merchant, and the weekends. The saving is often smaller than it looks, and occasionally negative.
7. Consider splitting the job.The most efficient answer is often a hybrid: pay for the structure, the services and the certification, and do the stripping out, the making good and the decorating yourself. Agree the split with the contractor in advance so it is priced and sequenced.
8. Be honest about the finish you will accept.Tiling, plastering and joinery are the three where amateur work is most visible and most permanent. A wall you will look at every day for fifteen years is a different proposition from a garage floor.
9. Check the insurance position.Some home policies restrict cover for damage arising from the householder's own work, and self-installed work is often outside any product warranty. Ask before, not after.

Tips

• Doing the strip-out and the waste clearing yourself is the most consistently worthwhile DIY contribution to a professional job, because it is labour rather than skill.
• A half-finished DIY job is worse than none. Trades frequently charge more to correct one than to have done it originally, and they are entitled to.
• If you cannot describe how you would check the work was right, you cannot check the work was right.

Common mistakes

• Judging by whether the job looks difficult — Difficulty and legality are unrelated. A socket in a bathroom is no harder to fit than one in a bedroom and it is notifiable.
• Taking on buried work to save money — Waterproofing, membranes, insulation and joints in voids fail invisibly. The saving is realised now and the cost lands when the failure surfaces behind a finished surface.

Questions people ask