Jobs you should not DIY

Doing your own decorating, tiling or fence building saves money with nothing worse than a crooked line to live with. Some jobs sit on the other side of a line drawn by law and by physics, where the downside is a fire, a poisoning or a fall. These are the ones to hand over.

Gas appliances and gas pipework

Work on a gas appliance must be done by a registered gas engineer. That includes opening the combustion case, changing a fan or valve, adjusting the burner, fitting a flue, and any alteration to gas pipework or a meter connection. It applies to cookers, gas fires and wall heaters as well as boilers. Cleaning the outside of a hob or a boiler case is fine; everything inside is not.

Gas faults do not announce themselves. Carbon monoxide is colourless and odourless, and an appliance can burn badly for months before anyone feels unwell. No home test substitutes for a combustion reading from a calibrated analyser. If you smell gas, do not touch light switches or a phone indoors. Open the windows, get everyone out, and call the gas emergency number for your area from outside.

• Registered engineer only - In the UK, someone on the Gas Safe Register. Ask for the individual's number.
• Covered by the rules - Boilers, cookers, gas fires, flues, meters and all gas pipework.

Electrical work beyond the basics

Swapping a light fitting on an existing circuit, or changing a plug or faceplate, is within reach of a careful householder with the power isolated and tested dead. Everything else needs a qualified electrician. Adding a socket, extending a circuit, fitting a shower, replacing a consumer unit, running cable in a bathroom or kitchen, and any fault finding all sit the other side of the line.

Cables must be sized for the load, protected by the right device and routed where nobody will drill through them later. A ring circuit with a broken conductor can still power the appliances on it while overheating at the break. Old wiring adds its own problems: rubber insulated cable, aluminium conductors and rewirable fuse boxes need an electrician's eye before anything is added. Notifiable work also has to be tested and certified, and without the certificate you will struggle to sell.

• DIY-friendly - Replacing a light fitting, switch or socket faceplate with the circuit verified dead.
• Needs an electrician - New circuits, consumer units, showers, outdoor supplies and bathroom wiring.

Asbestos in older properties

Houses built or refurbished before the year 2000 can contain asbestos. It turns up in textured ceiling coatings, cement sheets on garages and sheds, floor tiles, pipe lagging, insulation board, and the panels behind some heaters and old fuse boxes. In good condition and left alone, most of it is not a hazard. It becomes one when it is cut, drilled, sanded or broken, because the fibres released cause disease decades later.

If you do not know what a material is, do not disturb it. Do not use a wallpaper steamer on a textured coating, since that softens the material and releases fibres, and do not attack a garage roof with a sledgehammer. A sample sent for testing answers the question cheaply. Sprayed coatings, lagging and insulation board need licensed removal, and even cement sheets need careful bagging and disposal at a site licensed to accept them.

• Do not disturb - No drilling, sanding, cutting or steam stripping of anything unidentified.
• Where it hides - Textured coatings, cement sheets, floor tiles, lagging and heater panels.

Roofs, ladders and heights

Falls from height are one of the largest causes of serious injury in home repairs, and the people hurt are usually competent with ladders. Working on a roof needs safe access, a way of spreading weight across fragile surfaces, and protection from a fall. A domestic ladder leaned against a gutter provides none of those. Old cement sheets can be asbestos and can crack without warning.

The same logic applies to anything above head height. Clearing a gutter above the first floor, cleaning a high window, fitting a satellite dish or removing a chimney chain are jobs to pay for. If a ladder is genuinely needed, keep it on firm level ground, angle it properly, have someone foot it, keep your hips inside the stiles and never overreach sideways.

• Pay for it - Roof repairs, chimney work, high gutters, aerials and flat roof access.
• Fragile surfaces - Old cement sheets and rooflights are the classic fall-through hazards.

Walls, chimneys and structural change

Knocking through a wall, removing a chimney breast, widening a doorway or cutting a beam into a floor needs engineering rather than enthusiasm. Walls carry loads in ways you cannot see, and older houses often use the chimney breast to brace the front and rear walls. Taking one out without a designed replacement can leave a wall bowing within a year. A structural engineer calculates the beam and building control inspects the steel before it is covered.

The same applies underground and outside the building line. Digging foundations, building a retaining wall next to a slope or excavating near a neighbour may require a party wall agreement or planning consent. Work done without approval surfaces at sale, when a buyer's solicitor asks for certificates you cannot produce, and it can leave insurance invalid after a structural failure.

• Load bearing walls - Need a designed beam and a building control inspection, not a judgement call.
• Paper trail - Certificates matter as much as the work when you come to sell.