Leon Smith and Katherine Nicholson can point to the exact line where their old house ended and the new part began.
But a visitor to the property would be hard pressed to distinguish the 2012 build from the part that was there 100 years earlier, as they were so meticulous about matching villa features from old to new.
The couple bought the bay-fronted villa on the crest of the hill behind Birkenhead five years ago.
"We started planning when we bought the house, but it needed heritage consent, lots of interesting conversations," says Leon.
"We followed the roofline of the old house when we built out the back, with a kink in the side - all to show where the new begins. We didn't want a modern glass box."
The couple - she is Canadian, he is from South Africa - made sure that the entire house was insulated and warm, with gas central heating.
They had the bitterly cold experience of six months living through the major part of the build - in winter, with twin toddlers - and did not want to be cold again.
The build progressed in stages over two years or more, but Katherine did insist that the master bedroom was ready for occupation when she returned home with new baby later in the build - a hurried working bee by Leon and his family saw to that.
Engineer Leon was very hands-on in the build, hand-turning each of the bannisters on the stairway to the upper floor master bedroom and creating a Parisian-style steel Juliet balcony off that room's french doors.
His reward was an office/man retreat in the loft of the new double garage at the back of the property (it can also do duty as private guest room for visiting family) and plenty of workshop room in the garage.
The re-design of the house focused on the usual villa problem - a rumpty series of lean-tos at the back of the house for kitchen and bathroom (the loo was on a back porch).
The couple were sad to be told that the original kauri floors were too rotten to be restored, so found a wide-plank oak floor board to give the vintage look they were after.
They were determined that the modern back of the house was not going to look like an add-on, spending endless hours tweaking floor plans.
They found an old sash window under the house to refurbish for the new bathroom, and continued the villa-style sash windows (albeit on an elegant, grander scale) in the new kitchen and living room addition.
Leon laughs that the bay windowed sun-room overlooking the back garden was the most expensive part of the build, as the couple wanted to replicate the shaped ceilings of an older conservatory, a complicated build.
It is now Katherine's favourite spot, where she can watch the kids playing in the newly landscaped back yard.
They kept the villa roofline on the addition, but by dropping the floor level down a few steps to meet the back lawn they managed to keep generous high ceilings (complete with old-style beading) in both upstairs and downstairs rooms.
The front of the house retains a classic villa format - generous bedrooms off a central hallway - and two of the bedrooms have original fireplaces (blocked off for insulation).
There's a generous family bathroom, complete with a stylish tub, and build in closets in each room (carefully embellished with period panelling and handles to fit the house).
The family room and kitchen were worth the winter of cooking behind tarpaulins on the front verandah: a great cooker, soft grey cabinets with beautifully panelled doors and vintage handles, a huge island bench for everyone to gather around, hardworking benches (stainless steel for him, marble-look engineered stone for her).
The walk-in pantry/scullery is where the old back porch loo was, but there is little clue as to the rough start the old house had, with its stainless steel benches and organised wall of shelves.
The real pleasure for Katherine and Leon is the huge master suite upstairs. As well as their sunny bedroom, where they added plenty of Velux roof windows for glimpses of sky and surrounding trees, there's an office space for Katherine, a walk-in wardrobe (that also has access to more loft storage) and a bathroom.
The tub is positioned under another set of skylights, so that Katherine can gaze at the stars when she lies in the bath. Leon can point to many of the finishing touches he built himself.
But the family are keen to give their children a truly country lifestyle, complete with bush and streams and room for the three dogs to run around, so are selling their villa to move out of town.