Joining to existing footings

[TURNWISE]

I am building a new conservatory. One wall of the conservatory will be 2.1m high and will be tied to an the external wall of the house. The other walls will be 600mm high.
The 2.1 high wall already has footings, as when the house was built the builders erected a garden perimeter wall.
We intend to reuse these foundations to build our 2.1m wall on but need new strip foundations for the other two walls of the conservatory.

My question is do I need to join these new strip foundations to the existing or is a simple butt joint enough.

The construction is exempt from building regs but I want it done correctly.

Please help

[stoneyboy]

TURNWISE,
Butt join your foundations and use concrete blocks below ground level ensuring that a complete block bridges the join.
end

[uk's best builder]

when butting up to old foundations you should apply a slip membrain