How does it effect my will?

The terms "joint tenants" and "tenants in common" are used to describe the way in which two or more people can own a property. (They have nothing to do with tenancies or leases.)

If two people own a house as "joint tenants", then, when one of them dies, the other person will automatically become owner of the whole house (regardless of the terms of the deceased owner's will). A married couple will usually choose to own their home as joint tenants.

If, on the other hand, two people own their home as "tenants in common", each person only owns a share in the home (usually quantified according to their contributions to the purchase price). When one tenant in common dies, his or her share will pass into their estate and be dealt with by the personal representatives, either under the terms of his or her will or according to the so-called rules of intestacy. (You will see, therefore, that a tenant in common can leave their share in their will, unlike a joint tenant.) A couple who buy a house together and who are not married will usually choose to own the house as tenants in common.

or, put another way...

Property registration in which two or more individuals own a certain proportion of a property. Each person's proportion is distributable as part of the owners estate, so that if one of the account holders dies, that owner's heirs are entitled to that proportional share of the account.

Parties in a stable relationship, but unmarried, also often choose this way of holding the property, but if this applies to you, you should only do this with care. In particular, where people come to a relationship with children by a first marriage, a joint tenancy can disinherit the children of the spouse who dies first. His or her interest will pass straight to the survivor, and the children will then be dependent upon gifts made by the surviving spouse (who may again remarry). This cannot be controlled by a will very easily - property held under a joint tenancy can not be disposed of by a will.

In a joint tenancy, whatever the proportions in which the parties have actually contributed to the purchase price, and/or to the maintenance of the property or mortgage, the only safe working assumption is that any proceeds of sale will be divided equally. To put a house in joint names is to make a gift of any excess contribution to the other party.

Tenants in common each own a specified share of the property. When one owner dies, his or her share falls into that person's estate and passes according to their will, or as on intestacy. This mode of tenancy can sometimes be used as part of estate planning (trying to pay as little tax as possible). This is much the better way to hold property in all circumstances where a joint tenancy is not immediately and obviously appropriate.

For tenants in common it is sensible and sometimes necessary, both to make wills, and also to be crystal clear about the precise agreement between the owners on such matters as the proportions in which the property is owned, who can live in the house, who decides when the property is to be sold, and so forth. We can advise you about the issues you should consider. The agreement is then best recorded in a formal trust deed. As an absolute, and inadequate, minimum, the details should be recorded in writing and signed by each of you.

As you currently own your property as Joint Tenants on the death of one of the owners, the property would then pass automatically to the surviving joint owner and cannot be controlled by a Will.  However, changing to Tenants in Common allows you to control the destiny of your share.

When changing to Tenants in Common it allows you to each own a share in your property (ie 50/50 or 70/30 etc) and then to gift the property on your death whilst giving the surviving spouse a lifetime interest in the property.  This means that your spouse can live in the property for a lifetime or for a specified period or with a proviso (ie until the children reach a particular age or until your spouse remarries).  You can also have a say in whether the survivor is allowed to sell the property and upgrade or downscale.