Got it working perfectly thanks !

So the only way is to define a constant max size for this buffer inside the code...?

Worst case would be 3 x numInstances which could add up to quite a lot.

By the way I'm currently assigning each instance the 3 float4's inside a loop. I assume it would be more efficient to add them inside a structure which I then copy once using memcopy or is that neglectable?

I'm only doing map / unmap once, but I'm filling the data like this:

// Update buffer
XMMATRIX worldTransform = XMMatrixTranspose(XMLoadFloat4x4(&scenePrimitive->worldTransform));			
for(int u = 0; u < 3; u++)
{	
XMStoreFloat4(&pInstanceData[(numInstances * 3) + u], worldTransform.r);
				}

basically in the loop that goes through every instance and counts them to do the instanced draw call at the end.

Also am I correct in thinking that I am restricted to one specific material (texture and properties from const buffer) for every unique instance group ?

update: Just read about putting them inside a Texture2DArray...so for specific material properties would it make sense to also store them inside a buffer like I did with the transforms ? Might be overkill for e.g. booleans but still...

I only use a vertex buffer yes, not using instance buffer at all but giving it per instance transformations via a float4 buffer that holds these.
They're then accessed inside the shader using the instanceID.

Hmm I do remember having some issues that no pixel shader was being executed....can't remember exactly what it was though...

Have you made sure that your InputLayout is correctly set ?

Should look something like this:

	D3D11_INPUT_ELEMENT_DESC lo[] = 
	{
		{"POSITION", 0, DXGI_FORMAT_R32G32B32_FLOAT, 0, 0, D3D11_INPUT_PER_VERTEX_DATA, 0 },
		{"TEXCOORD", 0, DXGI_FORMAT_R32G32_FLOAT, 1, D3D11_APPEND_ALIGNED_ELEMENT, D3D11_INPUT_PER_VERTEX_DATA, 0},
		{"NORMAL",   0, DXGI_FORMAT_R32G32B32_FLOAT, 2, D3D11_APPEND_ALIGNED_ELEMENT, D3D11_INPUT_PER_VERTEX_DATA, 0},
		{"TANGENT",  0, DXGI_FORMAT_R32G32B32_FLOAT, 3, D3D11_APPEND_ALIGNED_ELEMENT, D3D11_INPUT_PER_VERTEX_DATA, 0},
	};

fourth parameter is important here as this is the vertex buffer slot...

Also try to make sure the float4s are combined in the correct order.

I've had a lot of problems with this. In the end I had to transpose the matrix first and then pass them to the cbuffer because otherwise you'd loose the 4th row because

if you pass your 3 rows something like:

cbufferData.data = worldMatrix.r;

you're passing only 3 vectors of this matrix and you will loose important data if you pass the rows instead of the columns.

That's why I changed it over to columns and it worked:

float4x4 GetInstanceTransform(uint instID, uint offset)
{
	uint BufferOffset = instID * elementsPerInstance + startIndex + offset;

	float4 c0 = InstanceDataBuffer.Load(BufferOffset + 0);
	float4 c1 = InstanceDataBuffer.Load(BufferOffset + 1);
	float4 c2 = InstanceDataBuffer.Load(BufferOffset + 2);
	float4 c3 = float4(0.0f, 0.0f, 0.0f, 1.0f);

	float4x4 _World = { c0.x, c1.x, c2.x, c3.x,
						c0.y, c1.y, c2.y, c3.y,
						c0.z, c1.z, c2.z, c3.z,
						c0.w, c1.w, c2.w, c3.w };


	return _World;
}

I don't see a way around that...but maybe someone does ?

Yes but that only applies to float4x4 values coming from the cbuffer. It doesn't apply to what you use inside the vertex shader and also not for float4's.