The number of Scottish homeowners taking in a lodger has soared by 19% in just two years.

Aberdeen has seen the biggest rise - up 140% in two years, according to SpareRoom, the flat and house share website. The average monthly lodger rent in the city is £512, including bills, and there are three potential lodgers searching for every available room.

The lodger market is also healthy in Edinburgh, where the number of new 'lodger wanted' ads has risen by 8%. In Glasgow, there has been a 5% increase.

The trend for homeowners letting spare rooms is now growing by almost a quarter (23%) year on year across the UK, as people look for new ways to boost their incomes.

The average resident landlord in Scotland earns £4506 a year and many use the money to fund their mortgage: almost half (45%) of live in landlords couldn't afford to pay their home loan without a lodger.

Graeme Brown, director of Shelter Scotland, says:

"Increasing numbers of homeowners are looking for innovative ways to maximise their income. For many, the experience of renting out a room in their home can be a positive one and can provide a welcome stream of additional income."

If you want to join the growing band of resident landlords, you should first tell your mortgage lender. The mortgage contract will probably allow for a lodger, but there might be certain conditions. It's also important to inform your contents insurer. You might have to pay an increased premium, but if you don't tell the insurer about a lodger you could invalidate the policy and it would not pay out in the event of a claim.

Bear in mind that the income from a lodger could affect your benefits or tax credits. If you live alone, you will also lose the 25% discount on your council tax. However, you can charge the lodger to cover the loss.

A lodger would usually be a common law tenant, so their rights would be set out in the tenancy agreement. A common law tenancy does not grant the lodger as many rights as an assured tenancy or a Scottish secure tenancy, but the agreement should state the rent and what it includes, such as bills and council tax, the house rules and the lease term. You can download an example of a lodger agreement from Shelter Scotland (www.scotland.shelter.org.uk).

Tristan Compton, the National Landlords Association's representative for South East Scotland, says: "A written tenancy agreement is essential to protect both the landlord and the lodger. We would also recommend that a landlord joins a professional organisation such as the NLA so there is somewhere to go for advice in case anything goes wrong."

If you are a resident landlord, you don't have to register any deposit with one of the three tenancy deposit schemes approved by the Scottish government. But you do have to make sure that your home is in a fit and proper condition. It should be clean, in good decorative order and free from health hazards, such as vermin. Furniture must also comply with fire safety regulations and a Gas Safe engineer must carry out an annual gas safety check.

Rent a room

Resident landlords can let rooms to lodgers and earn up to £4250 a year tax-free under the government's Rent a Room Scheme. But there are certain conditions. The accommodation to let must be furnished and in your only or main home.

If the gross income from the lodger, before the deduction of any expenses, does not exceed £4250, the landlord does not have to worry about tax. But if the gross income exceeds the threshold, the landlord can choose either to pay tax on the actual profit - in other words, the rent minus any expenses and capital allowances. Or, the landlord can pay tax on the gross income minus £4250, without any further deductions.

The tax threshold has not changed since 1997, which means that 40% of Scottish towns now charge average lodger rents that exceed the allowance. Some experts therefore want the government to raise the threshold.

SpareRoom's Raise The Roof campaign is supported by Shelter and the National Landlords Association and calls on the government to increase the Rent a Room Scheme tax-free threshold to £7500 a year - and raise it annually in line with inflation.

Matt Hutchinson, director of Spareroom.co.uk, says:

"The current £4250 threshold discourages people from letting rooms because they're nervous about having to fill out tax returns. Increasing the government's Rent a Room Scheme limit to reflect the market today, rather than 18 years ago, will act as an incentive to many cash-strapped homeowners."

City/Town Monthly Lodger Rent

Aberdeen £512

Dundee £308

Dumfries £329

Edinburgh £436

Livingston £373

Falkirk £381

Glasgow £407

Inverness £408

Kilmarnock £351

Kirkcaldy £335

Paisley £298

Perth £368